At the moment, spell books are too cheap. There should be more books, with more varied spells, not just slightly different clones e.g. Ice Bolt, Fire Bolt. The books should be more varied, and have more spells.
Choosing Fire over Ice should make more of a difference to game play, not just the colour of the magic bolt. In addition books should be more expensive so it entails a major sacrifice to choose them all - if that is even possible.