What books do you think are the most useful when getting into advanced theory? I want to get a book on harmony, counterpoint, and chord progressions. I know a lot of people like Piston books but they are pretty expensive, but if they are a lot more useful than the others I can get them. What're some of your guys favorites?

PS: I'm interested in learning of the techniques of any period from the common practice era.

Edit: I just started playing piano so if they have any sheet music that isn't doesn't require any virtuosity (wrong word?) that would be fantastic.
Patrick Star of The Jelly fishing Club PM darkstar2644 to Join
TREATISE ON HARMONY by Jean-Philippe Rameau. good breakdown on points covered