It is generally best practice to commit to using only one Grimoire as your main weapon while keeping a second Grimoire ready. Everytime you equip a side book to a Grimoire, you gain Magic XP which increases your overall magic proficiency. Since this is the only way to gain Magic XP, it's best to dump any side books you don't want your actual Grimoire to have onto a second Grimoire that you're not going to use anyway.

Risky side books increase your Grimoire's damage output significantly while adding some potentially dangerous quirk. However, some of these quirks can be made negligible through some counter measure.
If you want to get somewhere in this game (as in, defeat the secret boss or beat Expert/RED difficulty), you will need to start making use of these.

Coin Toss Spell: Power increase, but lose coins with each shot.
Note that you lose coins per shot. Equiping side books that increase your shot count, increases the rate at which you lose coins too. Only recommendable if you really do not plan on buying anything for the remainder of the run.

Gravity Spell: Power up. No knockback while shooting, slower movement.
Equip this side book onto a Grimoire that has firing cooldown, then use your Grimoire by tapping, rather than holding the Shot button. The character only slows down during the frames that your Shot button is held but the Grimoire still cools down outside of that.
Unless you're using a Grimoire that requires holding Shot (like a Plasma Beam), I would highly recommend this side book.

Left Spell: Power increase. Fired bullets curve left. More curve with stacks.
This can be cancelled out with Right Spell.

Right Spell: Power increase. Fired bullets curve right. More curve with stacks.
This can be cancelled out with Left Spell.

Double Edge Spell: Power increase, but bullets hit self too.
This power is fine as long as your Grimoire does not have a Mana Retreat or a Wall Reflect side book equipped.

Bowling Spell: Increases power, but bullets can fall into holes.
Counter measure: Use a second Grimoire if you need to fire at an enemy on the other side of a hole.
Personally, while this spell is not bad, I avoid using it altogether, since I prefer to focus on only having to use my main Grimoire but it is okay to use on lower difficulties where enemies still give you enough time to think.

Lava Conversion: Power increase. Bullet speed down, penetration up. More effect with stacks.
Bullet speed doesn't just simply go down. It goes way down. Personally, I would never use this one.

Blazing Sun: Power significantly up, but lose bullet control.
(WIP)

Final weapon: Delayed firing, uses all mana, but power up.
(WIP)

Inorganic Specialized: Damage dealt to inorganic-type enemies is increased, but damage dealt to monster-type enemies is decreased.
Counter measure: If you have the Magic-Generalization side book which nullifies negative modifiers, there is no cost here. However, I would not equip this otherwise.

Wave Spell: The bullet fired will fly in a serpentine pattern. It penetrates terrain.
(WIP)

Dark Spell: Power significantly up, when it hits, it forms a dark zone where nothing can be seen.
Counter measure: Equip a homing side book.
Aside from being a little irritating visually, it's possible that the dark clouds will make it difficult for you to be able to tell whether an enemy you shot at is still alive or not. Other than that danger, this is a good pick.

Anti-Living Magic SP: Damage dealt to monster-type enemies is increased, but damage dealt to inorganic-type enemies is decreased.
Counter measure: If you have the Magic-Generalization side book which nullifies negative modifiers, there should be no negative here. However, I would not equip this otherwise.

Delay Spell: Power increase. The bullet pauses momentarily after being fired, then shoots out belatedly.
(WIP)

Self-Decomposition: Power significantly up. When mana reaches zero, instantly restores mana in exchange for one heart.
In short: You lose a heart if you run out of Mana. This side book is a no-brainer if you have a Mana-efficient Grimoire.

Personal recommendations
My weapons of choice: Magic Shot, Spirit Arrow, Mortal Bullet
Risky side books:

1. Dark Spell
   
2. Gravity Spell
   
3. Self-Decomposition (risky with Mortal Bullet due to high MP consumption)
   
4. Double Edge Spell (as long as there's no chance of your own shots hitting you)
   
5. Bowling Spell (not recommended on Red difficulty)

Within a few frames of being hit, you can use a bomb in order to negate the damage. This can even negate the fatal final hit.

I could not find an official name for these, but on stage 4 and 5, some enemies will spawn green ghosts upon defeat. These will follow you around and remove a random side book effect from your current Grimoire upon touching you, though the actual removal will be delayed until you're done with the enemy room. "Spell broken" does not mean that you avoided the effect, but that that's the moment the affected side book has been removed.



There are multiple ways to deal with this:
1: Simple dodge them. If you roll through them, they will need to make a U turn, so it's actually quite effective to dodge them by doing either that or just barely moving past them. However, these ghosts will persist for a while and there can be multiple too, so this can become annoying real quick.
2: Destroy them with a bomb. Luckily, these ghosts can be cancelled with bombs. However, this obviously uses up a valuable resource.
3: Get hit intentionally. This is where having a burner Grimoire is helpful once again. If you simply get hit after switching to a Grimoire you don't actually use for combat, you don't lose anything of value when getting hit by the ghost.
4: Destroy them with the Dodging Manual III artifact. If you've acquired this manual, you can roll through green ghosts to destroy them.

The pattern consists of large orbs and small needle bullets. All of these bounce off of the room's walls and remain on the screen for a while. However, all of the small needles are fired right at the start.

If you use a bomb at the right moment at the start, they can all be cancelled at once and you'll only have to dodge the large orbs that exclusively move vertically or horizontally for the rest of the countdown.

If you check out this guide made by the very dedicated player エルドラ, you'll find a bunch of information. While it is in Japanese, I highly recommend checking out the last section, which lists the possible dungeon layouts. For Stage 1 to 4, there are 3 different layout patterns. For Stage 5, there is only one that starts with two branches that are indistinguishable from another for multiple rooms.

Using this information is incredibly useful if you want to avoid entering unnecessary rooms.

Here's what the texts in the fields mean:
スタート: start
小部屋: small room
中部屋: medium room
大部屋: big room
ショップ: shop
宝箱: treasure room
禁断の書: forbidden fragment room
ボス: boss

Scoring actually has an overall benefit. Reaching certain score thresholds grants you additional dice that allow you to reroll side books.
You get one die for each of the following thresholds:

• 2,500
  
• 5,000
  
• 7,500
  
• 10,000
  
• 20,000
  
• 40,000
  
• 60,000
  
• 80,000

After that, you get two dice for every 100,000 mark you hit, meaning that you obtain new rolls at a much lower rate than before.

To my current knowledge, the basic scoring system mostly wants you to build a weapon that makes enemies produce as many score items as possible but this might go against building a strong reliable survival weapon.

However, you can also equip the skill which adds the combo system, an additional score factor that rewards you for killing enemies and not getting hit throughout your run. If you want to acquire a very specific build and raise your chances for getting it, make sure to do so!