To me one of the most important books on music composition came at the very dawn of the classical period, Rameau's Treatise on Harmony of 1722. There were several followup books through the 1750s, and I cannot say which is the best. (Nouveau système de musique théorique; Génération harmonique, ou Traité de musique théorique et pratique; Démonstration du principe de l'harmonie).
Rameau was at the time dealing with a limited repertoire -- most of the classical music had not been yet written, and Rameau himself is considered a late-Baroque rather than Classical composer. Yet the fundamental principles expressed in these books are an enormous and still-relevant achievement. The explanatory power of Rameau's theories cover a wide range of later music in addition to that of his own time, and music theory textbooks of today are arguably, in a large sense, plagiarisms of Rameau's writings.
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