Great game, I donated :).
Some expansion would be definitely cool.
I have some minor suggestions:
1. During the fight, it would be handy to see right away how many spells does each book have left (maybe instead of book number (which is superfluous I think - book position is evident) or in addition to it (with some other color?)). Currently I have to pause and scroll through all the books to find out how many spells are remaining.
2. When I hit space during visual effect of attack, a bat curse etc., the game does not pause. Instead it could remember that I've hit pause and pause after the effect ends.
3. At final statistics show also gold earned.