That, combined with the fact that mages (the only class that uses spellbooks) can't use armor or most weapons, have horrible survivability in general, don't get familiars to help them out, and that probably the one saving grace of mages (Weavecrafting/summoning) is currently game-breakingly borked, there's really not much reason to pick them. Druids are superior in pretty much every way.
... do I really have to go and do another mage challenge? The last one used detect monsters and floating disc as its only persistent spells (unless you count levitation, anyway). That was
it. I think it had to restart once, twice, before winning? Mages are ridiculous. Mind you, druids are
too (I consider druids the easiest class in the game, m'self), but mages are like in second place for survivability and ahead in sheer numbers of ways to break the game over their knee.
Summoning
is sillily powerful (for anyone -- bard, priest, mage, druid, whatever), but mages have ridiculous amounts of tools even without them (or using them in limited quantities). Phase door alone is godly, the oodles of utility/debuff/LoS control junk they have, the whole junk-like-minor-drain thing, necromancy minions in general, illusions (easily gamebreaking, even without everything related to them that's not implemented. Nothing short of wands can generate minions as powerful as illusion spells.), the huge swathes of ranged offense available... they're (somewhat) squishy, but that's what rolling high con is for. And all that's without getting into prestige stuff -- loremasters can get the highest spell DCs in the bloody game, alienists are nigh broken (admittedly, it's even worse with druid alienists), there's... just this huge list of ways in which mages are easily the top one or two classes in the game (competing with druids) in terms of... basically everything.
Yes, they can get knocked over pretty easily if something gets in melee with them, but
nothing else in the game is as capable of making sure that
never happens.
I could rant further, but yes, druids and mages are easily tied for the most powerful classes in the game, followed by priests and then
everything else. Druids are easier to play (and potentially win with, especially once you get the
utterly overpowered earth meld), but they and mages are definitely tied for the game's top-tier doomcritters. Incursion does a fair amount to give non or partial casters some love, but it is still most definitely in the quadratic/linear school of power. There's mages, druids, priests, and then the rest of them
...
As for spellbooks. There
is a chance you end up not getting a few neat spells before the game's over, but it's... pretty low. Libraries are common, other casters are common, the starting store almost always has a few keystone books in it, chests are freaking drowning in the things. Spell access is almost never an actual issue in practice -- even if you don't get something you were banking on, you're almost guaranteed to get something that works just as well. The only thing I was talking about in regards to the post above that was re: a specific spellbook that isn't
implemented, and holds a couple key illusion spells (note: Illusionists don't even remotely need them to win. All they need is shadow monsters, and that's implemented). By the end game, you'll almost always have ran into multiple copies of almost every single book that's actually in the game. The dungeon is ruddy stuffed with spellbooks.