[Radek]

Right. Using books does not "spoil" the character at all. First of all, you get your mana points every time you add a spell to your spellbook no matter how many books you have used. Second, the game seems to control your mana points a similar way it controls your hitpoints. As a result, we get the following:
1. If you do everything possible for gaining as many mana points as possible in the beginning the game, then you will have to wait for high level mana point increases a long time. For example, up to level 50.
2. If you "spoil" the character by using books and bypassing the trick with collecting mana points by a barbarian, then you will be given high level mana point increases much sooner.
The resulting number of mana points is the same in the end. The only difference is that the "spoilt" character will have considerably less mana points temporarily. That's all.
An example from my investigation: A "heavily spoilt" character - about 990 mana points in each magic school at the end of the game. A carefully optimized character - 1028 mana points in each magic school at the end of the game.