There are a large number of skill books, some of them very easy to miss. This guide will list out the skill books with a brief description and I'll list what skill trees it's associated with.

Disclaimer: Translations and naming are based on my local translation so may differ to what is on your end. Screenshots are usually cropped in a way to easily get under the file size limit.

Some books I've gotten in 5 plays in a row and then on the 6th it doesn't happen like it did all the other times before. What changed? I don't always know, but this guide may not have all the answers if you are expecting to get a certain book and don't know why you can't.

• Bone Splitting
  (Clever). Shut down the Black Shop west of Shijia Village.
  
  
• Corpse Venom Blood
  Poison 80. Found on the ground after solving the muder at Beautiful Dream Pavilion.
  
  
• Cotton Palm
  (Quiet, Soft). Use a jade coin on a statue. Both halves of the coin can be found east of Gongshi Village. One by giving some booze to some workers (given to you in a bag), another by fighting some thieves that raided a cart. Assemble the coin by clicking on each half. Statue is below.
  
  
• Dark Cold Spell
  (Active, Clever, Soft) If your village reputation is high enough when you save Gongshi Village, the chief will give you a key and a quest marker to follow. Make sure you do all other quests firsts for enough reputation.
  
  
• Dirty Three
  Must have an evil personality. Talk to the outcasts outside Gongshi Village.
  
  
• Drunk Flowers
  Intelligence 60. Get from Di'er Chi at the Fuxian Academy. Get it after the boat fight with Yan Yushu, but before you meet her in the pavilion.
  
  
• Duan Jiaxin
  Recruit Duan Siping for the second time after defending his home.
  
  
• Flower Fist
  At Tianlong Temple in the middle of the day, choose the second dialog choice to the monks cutting the child's hair.
  
  
• Goddess Fu
  (Clever). Get automatically when Fan Xiangdie joins, or steal it from her early.
  
  
• Hawk's Claws
  Found in the cave under White Horse House as part of He Yuqing's quest line.
  
  
• Iron Sand Palm
  (Hard). Traded by the blacksmith in Gongshi Village. You need to give him three sheets. First is on the roof. Second is under some bags that the nearby worker pays you to move, but you need a certain level of village reputation before you can start it. Third is in the hay by the chickens but can only be retrieved after you fight the thief at night.
  
  
• Mysterious Palm
  Fight or recruit Chu Xianglian.
  
  
• Nuchuan Fish Palm
  (Soft, Stupid) After you restore Zhou Yi to power in Shu Yuji, go back to the statue by his jail cell. You need to unlock the proper order: right, back, left, front. This gives you negative city rep but also addds to Fist and Blade skill.
  
  
• Stained Clothes Eighteen Falls
  (Quiet). Narrow window you can get this. It must be before you recruit He Yuqing after you've had the 3-way duel in the Shu Yuji tavern. Talk to the beggar at the exit on the north end of Dayan Town and pay him some money. He'll lead you to some bandits. You need to execute them after the battle and search the body.
  
  
• Valley Finger
  Get automatically when Cheng Yanhua joins, or steal it from her early.
  
  
• Water Margin Palm
  (All). Starting skill book. Can be exchanged for at Fuxian Academy.
  
  
• Wild Ball Punch
  Starting skill book, protagonist only.
  
  
• Yuejia Sanshou
  Found in a box at Qingtian Village. Grab it at night.

• Chunjian Formula
  Get automatically when Qi Xiao'er joins
  
  
• Drunken Eight Immortals Sword
  (Active, Clever). Get 6 types of wine for Xu Xiake. Of the 6, the notable that you must find are the wine on the remote eastern island with Monkey Three and also monkey wine which you can get from the tiger pit southwest of Dayan Town. This is a great skill book for a strong 3 energy attack but most of the book is locked behind the talent tree requirements.
  
  
• Dust Sword
  Top of tower near the tea house.
  
  
• Duan's Sword Spectrum
  Get automatically when Duan Siping joins.
  
  
• Five Thunder Penetrating
  (Hard, Active). Find in the ruined temple as part of the Zhang Junbao quest chain. The skill book is in a pit of snakes but just jump behind them to loot it.
  
  
• Guangan Sword
  (Soft, Active, Hard). Get automatically when Zhun Sunning joins.
  
  
  
• Hundred Flowers Sword
  Agree to ambush the two white robed swordsmen in Dayan Town. You first see them at the blacksmith. If you dont want to ambush them you can steal it from them by pushing them into a wall on their walk from the smith to the inn.
  
  
• Hunjin Sword
  (Hard, Stupid). In the Barracks. Can only get at night. Can just walk past the guard for some reason.
  
  
• Moon Sword
  90+ Intelligence. Get automatically when Yan Yushu joins.
  
  
• Pine Sword Spectrum
  (Clever, Active). Second floor of the Gonshi Village inn. Notable for the early segment attack and the solid 3 energy attack.
  
  
• Rainbow Sword Method
  (Clever, Hard). Save He Hongyuan from being attacked west of Dayan Town.
  
  
• Soft Cloud Sword
  Get automatically when Zhang Junbao joins.
  
  
• Winter Sword Spectrum
  (Soft, Quiet, Stupid). One of the few double sword skill books. Must be done before you clear the snake cave. Step 1: get a box. Step 2: put her head in that box. Step 3: steal the book.
  

• Bloody Blade
  (Active, Soft) Evil 50+. First you need the foreign monk quest from Wanglong village in the northwest. This is after you clear it of barbarians and spar with soldiers. Then at night, clear the four surrounding camps of monks. After that, they will ambush you in the morning south of the village. If you want the blade itself, go to the cave during the daytime and rest until night to steal the weapon, though difficulty is high. It should be the second item.
  
  
• Blue Sky Blade
  (Hard, Stupid). Blade 80+. Given to He Yuqing after the death of her father. Can also steal this early once he appears in Dayan Town.
  
  
• Broken Jade Blade
  (Hard, Active, Clever) After you restore Zhou Yi to power in Shu Yuji, he will let you ask for a boon. Ask for a skill to get this.
  
  
• Cut Mountain Blade
  Get automatically when He Yuqing joins.
  
  
• Five Tiger Broken
  (Stupid, Hard) Book three of the Beauty chain.
  
  
• Four Elephants and Six Blades
  (Quiet, Soft, Stupid, Hard) The second book from Beauty chain. Note on this one that if you just let the guy leave then you won't get the book. You either have to fight him, or go talk to the girl he's interested. If you talk to the girl you also get a bonus to your Blade skill.
  
  
• Great Wave Blade Spectrum
  (Hard, Stupid) From part four of the Beauty chain.
  
  
• Invisible Three Power Blade
  (Active, Clever, Hard). Quest chain that starts at the Beauty Restaurant. You basically follow this guy around defeating people.
  
  
• Nishikawa Moon Blade
  (Active, Hard, Clever). Half the tree is locked behind Active. Get automatically when Li Tan joins, or steal it from him early. Can also steal a copy from the guy who shows up later at the beggar camp north of Tibetan Flower Shed.
  
  
• Paoding Jieniu Blade
  (Active, Clever) From part five of the Beauty chain.
  
  
• Shock Wave Blade
  (Quiet, Soft) From part four of the Beauty chain.
  
  
• Thunderbolt Blade
  (All) Starting skill book.
  
  
• Tianpeng Blade
  (Hard, Clever) Blade 80+. You can get this one without any amount of combat even as early as day 1. Go to the cave entrance in the first screenshot. Once in the cave take your first right, then go left and up until you get to the book.
  
  
• Yellow Sand Blade Spectrum
  (Hard, Stupid). When your reputation is high enough and you buy a night at the Gonshi Village inn, the cutscene will play and someone will leave you a note to challenge you. Go up the nearby hill and challenge the guy, getting the book on a win. This skill book is outclassed in every way by Thunderbolt Blade.
  
  
• Yinfeng Blade
  You'll recognize this book as the one all the men in black use. You can purchase this skill book in Shu Yuji if you have an evil reputation.
  

• Dragon Whistle and Cloud Spear
  (Hard, Active) Spear 120+. Skills are locked behind talent trees. Get this by turning in 40 copies of Ancient and Modern Opera to Northern Clown once he appears at Beauty Restaurant.
  
  
• Eight Diagram Pole
  Get captured when speaking to the woman at the Black Shop west of Shijia Village. Book is in the jail cell.
  
  
• Eighteen Leopard Spear
  Spear 70+. Once you clear Gonshi Village, make your way to Baowang Village in the southern mountains. There you see a conversation betwen the brother and the village leader. Fight them and force the brother to tell you the secret. Then head up and west to the mine entrance. You'll fight some miners, get some darksteel, and will be attacked when you leave. You'll get a key from that fight. Go back to the room in the screenshot and unlock the door.
  
  
• Lingyun Fuxi Spear
  (Active, Clever) 70+ Spear. Dropped by killing E'Si Bo in Dayan Town. You must betray Su Qingrul when the time comes up. This also results in the death of several major characters.
  
  
• Middle Level Spear
  There is a fisherman on a cliff northeast of Gonshi Village. Speak to him and walk away, triggering a cut scene when he is killed. Talk to him before he dies to get a note to his family. Proceed with the quest and when the guy shows up at his family's house, stop him. Note that if evil then you cannot stop him or get the skill book this way. But the skill book is also in the same chest as the southern dragon ball above the cannibal bear cave.
  
  
• Night Fork Spear
  Get automatically when Luo Yuanyu joins. Defeat him in combat to add him.
  
  
• Pear Flower Spear
  (Stupid, Soft)
  
  
• Taigong Fishing Style
  (Quiet, Stupid) Talk to the patrons outside the tea shop in Dayan Town about rumors. One of them will be a midnight fisherman. Follow the quest marker to find him. You need to be good and might even need a certain amount of reputation to get it. Delay talking to him if you fail to get it, or choose to just steal it.
  
  
• Taizu Staff
  (Hard, Stupid) Available for trade at Fuxian Academy. Also available if you help the three monks on the river north of the Academy.
  
  
• Three Magic Spear
  (Hard, Active) Spear 80+.
  
  
• Wild Demon Staff Style
  (Active, Hard). Part of a very long quest, screenshots included are the beginning and end. When you turn in the quest, go to the Dayan Town north bridge at night to help with the duel. If you miss it, this book is also in the Academy.
  
  
• Yuejia Spear
  Found in a box at Qingtian Village. Grab it at night.
  

• Divine Sword Techniques
  (Clumsy, Quiet) Quest reward for clearing the ruined temple. Given by leader of Qingcheng School. Notable for it's 1.5 distance minimum attack.
  
  
• Flying Eagle Arrow
  Bow skill book.
  
  
• Flying Sand Over Sky
  (Stupid, Clever, Soft) Arrow 80+. Fight the cannibal bear in a cave south of Gongshi Village.
  
  
• Hand Seal Throat
  (Active, Hard) Fight the purse snatcher outside of Dayan Town's south entrance.
  
  
• Insulate Arrow
  (Quiet, Soft) At Gongshi village at night go up to the house on the cliff and witness the guy messing with the bull. Ignore it and talk to him after to get the book. Note that you cannot get the book if you talked to the old man and got the quest beforehand. Don't worry if you miss it though, this book is also in the Academy.
  
  
• Meteor Flying
  (All). Starting skill book.
  
  
• Nine Dozens of Tianzi
  (Clever) Arrow 70+. Push the rock over the cliff east of Shu Yuji. An old man appears to ask to recover his golden armor. Then buy or steal it from the group of guards. Note that it is much easier to steal it before you push the rock over.
  
  
• Seven Row Arrows
  Dodge 60+. Bow skill book. Get automatically when A'De A'Ke joins.
  
  
• Storm Pear Flower
  (Clever, Active, Soft) Given to you automatically by Bailu Shuang while you are on the Wang Zhe quest line.
  
  
• Throw Sparrows
  (Active, Hard) Start the quest given by the guy in the Gonshi Village inn. At midnight a monkey will appear in front of the inn. Follow the monkey and finish the quest in whatever way you choose.
  
  
• Xiahou Archery
  (Active, Hard). Found in a box at Qingtian Village. Grab it at night.
  

• Hong Meng's Knack
  (Hard, Stupid) Trade other skill books to either Academy or the vendor in the Ruins of the Ancestors. Can be easily stolen from the latter.
  
  
• Penetrating Bone Point
  (Hard, Active) Once you clear Gonshi Village, make your way to Baowang Village in the southern mountains. There you see a conversation betwen the brother and the village leader. Fight them and force the brother to tell you the secret. Kill him afterwards to get the skill book.
  
  
• Red Sleeve Music
  (Clever) Get automatically when Fan Xiangdie joins or steal it from her early.
  
  
• Song of Everlasting Hatred
  (Soft, Quiet) While on the road you can be ambushed by the men in black who have escaped from the cave. They will attempt to call their leader who you fought in the intro. You need to let one of them escape, while making sure at least one stays alive. Once she makes into combat you can finish the fight. I've seen the trigger at all times of day in at least four spots. I've marked the three I know for sure in the screenshot. There is also one on the east road but I don't remember the exact spot. It isn't anywhere on the road, it is literally one of those places.
  
  
• Xuan Tianwu
  (Hard, Stupid) Short 70+. When you escort Wang Zhe back to recover Qianting Village, there are two guards on the western wall. Approach them to initiate combat and get the skill book.
  
  
• Yan Wang Tie
  This is arguably one of the strongest skill books in the game. It lets you auto-kill lower level enemies with a three energy backstab. A major downside of this skill book is that you can't get it as a good alignment or with higher poison resist. I don't know the exact number of poison, but you probably need under 40 to get it. Stay for a night in the Shu Yuji inn. With low poison you will get robbed of all your money but they'll drop the skill book on the roof. With higher poison it won't work and you wake up. If you are evil then you can buy it from the vendor in town.
  

I mix these up in my head, will need to do another run to get exact notes.
• King of Medicine
  Doctor 70+. At Tibetian Flower Shed, proceed with Cheng Yanhua's quest. After you collect the ingredients you'll be attacked. Fight off the attacker and add Cheng Yanhua back into the group. This will trigger a cut scene to get the book.
  
  
• Meridian Acupuncture
  In Shu Yuji you will encounter two men getting attacked. Rescue them first and a doctor will send you on a quest to collect ingredients. Collect them all to get the book. Note that you need to provide the Cinnabar, which can either be found breaking open rocks at the river or by stealing it from the second level of the southwest compound.
  
  
• Qianjin Fang
  At Gongshi Village, finish the quest from the blacksmith's wife. This requires getting various boar parts which you can fully get from the Hunter quest in the same village.
  
  
• Shennong Herbal Classics
  At Gongshi Village, exhanged the Snake King corpse to the medicine salesman, or steal the book from him.
  
  
• Yao Shi Yeji

Note on Poison, you'll need probably above 50 Poison skill on the protagonist out of character creation to be able to use these. Not recommended to attempt for beginners.
• Blue Sky Spring
  Poison 70. Get automatically when Shi Hongtu joins. Screenshots added of tianshui toad and blue eye golden scorpion location, both underground.
  
  
• Corpse Venom Blood
  Poison 80. Found on the ground after solving the muder at Beautiful Dream Pavilion.
  
  
• Five Toxic Classics
  Starting skill book. Can also get by giving away the Snake King in starter village.
  
  
• Secret Absolute Toxicity
  Poison 70. Get automatically on the Duan Siping questline when you clear Tianlong Temple. Snow spider and rainbow centipede locations below. You can also can a snow spider by successfully cutting the kids hair.
  
  

• Blood Drink Magic
  Internal 70. Morality -50. Free the prisoner from the Qiantian jail in exchange for this.
  
  
• Buddhist Lion Roar
  Internal 150. Break the object overlooking the temple in the picture below. It creates a book in your inventory. You need high Qualification in order to transfer the book to a skill book.
  
  
• Chui Luan Gong
  Internal 15. Talk to the guy below, cannot be evil.
  
  
• Clutch Consultant
  Internal 120. Trade in at Fuxian Academy.
  
  
• Duan Jiaxin Method
  Internal 20. Get automatically when Duan Siping joins.
  
  
• Hot Tea Magic
  Internal 45. Talk to the guard in front of the tea shop in Dayan Town and follow the quest. This leads you to an old man behind the temple. He sends you the hill and duels you. If you survive 3 turns you get the skill book. I had at least one game where he just gave me the tea automatically and I don't know why. Possibly due to personality or rep, will update if I know more.
  
  
• Infinite Heart Method
  Internal 0. Dayan Temple is at the northeast corner of the map. Just to the west is a broken down house with this skill book inside. At some point in the game the house will be filled with people but I think you can still get the book in the day time.
  
  
• Mixed Element
  
  
• Nameless Remain
  Internal 120. Get when you recruit or defeat Huang Shang.
  
  
• Peach Valley Immortality
  Internal 55.
  
  
• Pure Yang
  Internal 70.
  
  
• Righteous Song
  Internal 15. Found under a rock in Gongshi Village.
  
  
• Rock Break Strength
  Internal 30. At the snake cave, first rescue the man in the first area. He sends you to find his friend on the upper level. Investigate and bring back the black object. If you give it to him, he trades you this. If you choose to instead keep it for the +300 HP then you can still get this at Fuxian Academy.
  
  
• Six Yogas of Naropa
  Internal 60. Steal it from Sang Deng in the temple north of Dayan Town.
  
  
• Qilan Mountains Formula
  Internal 30. Must have a Wild personality to start the quest chain with the monk on the hill in Shu Yuji. You reunite him with his sister in town. He drops it on the ground after the meet. Just load the game if you pick the wrong dialog choice.
  
  
• Six Meridians
  Internal 80. Get automatically on the Duan Siping questline just before you head to Tianlong Temple.
  
  
• Taixuan Sutra
  Internal 150. At the Liquor Village on the Li Tan questline, choose the below dialog choice to get thrown into prison. On the wall is a puzzle. The answer is one-eight. (Those two lovely girls died because you wanted this skill book, I hope it was worth it!)
  
  
• Tai Qing Gong Qi
  
  
• Tell Sorrow and Beg for Pity
  Internal 35. Dropped by killing E'Si Bo in Dayan Town. You must betray Su Qingrul when the time comes up. This also results in the death of several major characters.
  
  
• Tenguan Gongjie
  Internal 50+. Quest reward that starts with Zhang Lihai in Lost Water Village.
  
  
• Triple Wave of Yangtze
  Internal 40. There is a cave that you need a certain Internal Power requirement to blow open. Look the skeleton inside.
  
  
• True Sutra of the Northern
  You can copy this from the Academy library if you have high Qualifications.
  
  
• Two Way of Distraction
  Intelligence 40-. Unlike the other two books at library, for this one you need low Qualifications.

• Of note, two skill books under Fist give Dodge, Hawk's Claws and Dark Cold Spell.
  
  
• Daxian Greed
  80+ Dodge. Mandatory for anyone who wants to steal. On the roof of White Horse House is a note. Read it and then talk to the farmer to get a seed. You need to feed the horse a carrot. You can either plant your own or steal one. Planting your own is the ideal choice since it comes with a Fruit of Wisdom but it does takes 5 in game days to grow. Such as Day 1 Noon -> Day 6 Noon.
  
  
• Flying Geese
  An old woman in Gongshi Village thinks the gods are angry at her but it's just the kids stealing her food. Chase the kids down and let them go to get this book.
  
  
• Hopper Method
  Given to your automatically in the main quest line by Northern Clown.
  
  
• Lingbo Feixue
  Referred to as "Limbo Deluxe" in my head. Get automatically when Li Tan joins or steal it from him early. The Fist skill here is fantastic.
  
  
• Lingo Microstep
  You can copy this from the Academy library if you have high Qualifications. This skill book + Leopard Stew lets you hit 15 Internal Power for Yan Yushu to start picking up Internal books.
  
  
• Ruyang Knack
  Given by the little girl in Zhangren Village. Quest chain is something along the line of: talk to the smith, talk to the woman by the tree, tell her you know she is interested in the smith, go back and forth between them and the little girl until she runs away. Talk to the smith and go after the girl up the hill to the northwest. This also unlocks an armor blueprint.
  
  
• Tiannan Power
  Get from Duan Siping's brother when you dual him at the complex in Dayan Town. Locked behind a specific choice of dialog. Choose the first option below.
  
  
• White Ape Arrow
  Given to you by Monkey Three on the hidden island far to the east. Start on the island with the monkeys south of Lost Water.