My knowledge of music theory is very limited; I'd really like to get into some of the hardcore stuff. I've already read Tonal Harmony with an Introduction to 20th century music, which pretty much covers theory I and II ( as far as I know... ). What else is there to know? I'd really like to get whatever textbook is used for theory III, IV, etc. Also, I'd be interested in a book on rock theory if one exists... thus far none of the rock theory books I've read are at all serious, they're intended primarilly for the beginner guitarist and not for someone interested in music theory.