AFAIK you can find books for all spells of lv1-5(except lv1 mind stab), plus the one for lv6 banish. So there is about 30 spells to pick...if you are willing to take the time to lv up, since most people finish the game at around lv25.

the more spells you know, the higher your mana max. That is enough reason for me to learn as many as possible.

Good point. I learn as many as possible, but on levelling up, I select spells I know I *really* want. Then I buy/pick up every book I can get my hands on to supplement them.