I know what some of you are thinking. And what about sorcerers? Wouldn't this leave them in the dust? Well yes, it would, but don't worry, I have an idea for them too!
Sorcerers should have the ability to "spontaneously cast" spells. By this I don't mean just being able to cast their spells known in any combination they can afford, I think wizards should be able to do that with their prepared spells too. Sorcerers have inborn magical talents, power burning just beneath the surface yearning to be released, but difficult for them to learn to control. The spells that a sorcerer knows are those that he has learned to control, so he can cast those spells reliably. But he should have the option of reaching inside himself and drawing power that he hasn't learned to control yet too. This would be "spontaneous casting". A sorcerer could attempt to cast a spell he doesn't know, but doing so would require some kind of roll and carry other risks, like the spell might go wildly out of control if he fails, backfire on him, or do something else terrible.
This would fit the flavor of the sorcerer very well, and would give him the option to expand his versatility with risk, like a wizard with book-casting, but in a way that's different. The advantage would be that sorcerers could pull out just about any spell, not just those he has studied and copied into a book beforehand. The drawback would be that I'd make this riskier and perhaps more costly than book-casting.