Haha, I didn't read the OP carefully enough.
I kind of like the idea, but I share and into's concerns about the giant spell list. But I also share his view that the current situation is sometimes worse.
I guess you'd have to come up with some clever way to group the spells to make this list manageable. And ctrl+F should somehow search this list, so if I want to know if haste exists in this game, ctrl+F would need to search both shops, which contain books, and my available spell list.
I guess you'd have to slightly increase piety decay to compensate, after all, we won't be running back and forth to our book stashes all the time anymore.