Not directly books about composition, but these two books are fundamental reading if you want to understand (classical) music:
Charles Rosen, 'The Classical Style'. Fantastic analyses, and explains what art music is all about. Helped me understand much of the deeper meaning of music and art. Nicely written too, so it is a pleasure to read.
Hepokoski/Darcy, 'Elements of a Sonata Theory'. Revolutionized the way we understand sonata form. It's a voluminous theory book, so no easy read. But it's essential if you care about sonata form - the most important form in western music. Knowledge about form can help greatly for composition. In fact, I think that many times people get stuck and don't know what to do with a sketch or musical idea, it is because they don't know about musical form and thus can not use it's concepts to help them build a piece. Tough but rewarding read.