There are spellbooks. The Necromancy of Thay and the three books you can find in Sorcerer's Sundries as well as the book about dead gods from Act 1 all work as such and I do like the NoT quest. Though I wouldn't say no to more. The above mentioned are highlights of the game for me.
Instead of new categories, I'd prefer it if some of the lesser used categories would be explored more. There aren't any cool sickles in game apart from one, same goes for clubs or decent combat quaterstaffs.
