Are all the spells in the game able to be found via searching walls and such? Or is reading the manual to get spells also necessary. I'd think it'd be a lot more fun to hunt em down and it feels like the manual is giving em away. If so, is it the case for both UU 1 and UU 2 ?

Been playing a warrior (it's gotten much easier ... right now the biggest challenge is encumbrance, so I decided to jack around and make a Crocodile Hunter druid running around with his fists and trowsers using spells he finds and pummeling fools to death)

Simply put, reading the manual's necessary. The spells listed in the manual aren't told to you anywhere else, so reading the manual's pretty much the only way to 'learn' those spells, save by randomly experimenting with the runes.

However, there are some spells that aren't listed in the manual, which you can only learn in game.

This is the case for both Underworlds, if I remember right.

There are also several ways to learn those "unpublished" spells: some NPCs may tell you rune combinations; others can be deduced from finding several runes together; some are natural extensions of existing combinations (and/or intuited from playing the main series and knowing what the runes represent); etc. At least one 'hidden' spell was revealed by printing it on the box-cover screenshots!