Classical Form: Theory of Formal Functions for the Instrumental Music of Haydn, Mozart, and Beethoven

Caplin's book is, on the whole, an excellent revival of the Formlehre (study of form) tradition. His methodology of examining "formal functions" (i.e. how a particular musical module functions in the context of a full piece, rather than just labelling it) is incredibly revealing, and therefore his explanations of the various classical theme types (period, sentence, and small ternary) are likely the best you'll see in any study. The one caveat that I should express about this book is its reliance on the Riemannian and Schoenbergian traditions of harmonic theory: that's not necessarily a drawback, but to Americans who are used to the Schenkerian and roman numeral traditions of harmonic theory it can take a little getting used to (well worth the effort though).

This book is an in-depth outline of the four major forms of the classical era: Sonata, Rondo, Minuet/trio, and concerto. Before it covers them, however, covers the parts that make them, i.e. subordinate theme. Each of these small parts takes up about thirty pages, where the author covers tonal regions, structure, dynamics, etc. This book is a must for the music theory student/professional, or the composer.