Each dungeon has content at a fixed difficulty level. Obviously, since the maps are randomly generated as you go, there is some range to the difficulty. However, if you attempt to do higher level content at a very early point, you'll get killed for sure.

This guide is intended to aid you in deciding where to go next. I leave it open on a second screen while I play. I look at where I've been and decide where to go next. Usually, that'll be exactly what the guide says, but sometimes I vary based on my current situation. There is absolutely no guarantee that by following this order you will wind up beating the game without any challenge (I hadn't when I started this guide). It's a hard game.

Finally, I think I should point out that I play as the alchemist almost every game. In my experience, the alchemist's AoE is extremely effective in some circumstances that might otherwise be quite difficult for a 1v1 kind of brawler character. So, you should take all this with a grain of salt, and perhaps come up with your own order based on your character.

Consider this a soft guide for those who really just need something semi-useful to start with.

Thanks to ibanix for the link to the Recommended Zone Order Progession page of the Tome wiki: http://te4.org/wiki/Recommended_Zone_Order_Progression
Players should look that guide over, too.

If you look through this whole guide, or even to the next place to go, you're likely to find some content spoilage. Just warning you... if you don't want to spoil the game, then why are you looking here? If I ever come up with a decent way to avoid spoilers, I'll try to implement it, but I don't see how that's possible.

1.2 - Won the game and added notes. Cleaned up sections, adjusted stuff. Stuff.
1.1 - Added Mid-late and Late game sections. Adjusted some descriptions for clarity, simplification, or to make suggestions more universal.
1.0 - Initial Release based on my personal notes.

Someone named Row made a useful map for beginners. I'm including it here because it is great when you're first getting oriented. The world map is not randomly generated. However, a few events can pop up at random locations.
The forum post where Row posted this map:
http://forums.te4.org/viewtopic.php?f=41&t=34478

Row explains the color coding as follows.

Color coding:
brown - towns
green - tier 1 locations
yellow - tier 2 locations
red - later locations
pink - random locations
cyan - bodies of water

Trollmire (Consider coming back later for final boss)
Derth Arena
Norgos’ Lair
Heart of Gloom
<Dwarves> The Deep Bellow (in the Dwarf City, the Iron Council)
Scintillating Caves
Rhalon Camp

Lumberjack Village
Old Forest/Lake Nur (Watch out for those guys with energy shields (luminous horrors?). Come back later if you struggle to kill them.)
Derth Storm (lvl 14 after)
Kor’Pul (lvl 15 after)
Sand Dunes
Golem Grave Yard (Xafron Chi suggests waiting until later, perhaps early-mid; my experience suggest that the graveyard can sometimes be randomly much more challenging than it usually is, so consider waiting)
<Underground merchant guy>
Maze (lvl 19 after)

Last hope - learn armor, start brotherhood of alchemists quest
Last hope graveyard - Enter, but don't beat. This is one of a few areas with an adjustable difficulty based on when you enter.
Hidden Compound (turn off auto use before starting the slave brawl, if using the mod) (lvl 19)
Very old Halfling Ruins(lvl 20 after)
Ruined dungeon (careful, this one can be tricky)
Daikara mountains(lvl 23 after)
Temporal rift (level 24 after)
Undergound Kryl cult thingy (save Melinda and followup quests)(lvl 25 after)
Urkis (Derth storm continuation via that angel lady in the magic town)
*Farportal (lvl 27 after)
*Farportal, again.
Last Hope Graveyard
*Another farportal, if you can.

Dreadfell Tower (on my first attempts, I died three times at levels 26, 27, and 28.)
Volcanic Caldera (If it has popped up (30% chance). Wiki says 25-30, taking it on at 31 seemed quite safe)
Last Hope (central building)
*Far portal
*Far portal again
Gate to Reknor (up to portal)
Finish Reknor (Unremarkable Cave)

11x Orc Mobs (lvl 34+)

Gates of Morning (get the quest for Ardughrol)
Ardughrol (lvl 35 after)
Back door to the Vor Armory (If you aren't really close to level 36 by the time you get to the 2nd level, you might consider fighting some mobs) (lvl 36 after) (Also, the NW most room of the lower level is a surefire way to get yourself killed)
Gates of Morning (Get instructions to Briagh's lair)
Briagh's Lair
Gates of Morning

Last Hope (get Tannen's quest)
Reknor
Last Hope (get location of Telmur, Tower of Telos)

Backup Guardians - The wiki has a cool table for these guys. Just beat them in the order that the wiki has them listed. I'm attaching a snapshot of the wiki below. Note that the Reknor backup guardian listed in the wiki is the same one I just sent you to... I defeat him first since he is central to the plot.
http://te4.org/wiki/Backup_guardian

Elven Ruins (Lvl 40 after)
Telmur, Tower of Telos (lvl 41 after)

Finish up Brotherhood of Alchemists quest
Cavern leading to the Valley of the Moon (Near Rak'Shor Pride)
Prides - Although there seems to be little reason to pick one order over another, the wiki lists Rak'Shor as having very slightly weaker enemies than the others. So, I start there and complete the things near it, first. I end with Grushnak, since it leads directly to the next areas.

Do the first two prides:
Rak'Shor
Gorbat (Near Rak'Shor) (Lvl 44 after)

Then:
Erúan (Near Rak'Shor)

Finally, finish the last two prides:
Vor (lvl 46 after)
Grushnak

Slime Tunnels (Beneath Grushnak)

Everything else you want to do in the game should be done before entering the High Peak.

High Peak (from Slime Tunnels)

Feel free to comment on the order you prefer. Is there anything class specific that you take into account? Are there dungeons missing (some I'm aware of, such as the underwater cave)? Is there anything you think could be added to this guide to help other players?

Thanks, and happy times in the lands of Maj'Eyal!

*From SatanBless "Just a note to any players reading this, if you are playing on higher difficulties, especially insane, don't do Farportals after level 10, you will die there. Its very common for Out of depth monsters to spawn there, and the monsers are far tougher as well as more densely populated together in Farportals than regular zones. Don't throw your run away doing farportals on Insane to try and get some extra levels, its not necessary to win and will save you many hours."