Here is a short list of books about Music Theory, Composition, and Orchestration that will help to facilitate not only an understanding of music already composed, they will allow the reader to venture into the art of composition. Keep in mind, the information contained therein is not a sole means of writing music. A familiarity with multiple styles, listening, and some talent are also necessary components.

Some of the books on this list have become difficult to get but are well worth the effort!

Harmony by Arnold Schoenberg

Structural Functions of Harmony by A. Schoenberg

Fundamentals of Musical Composition by A. Schoenberg

Style and Idea by A. Schoenberg

The Schillinger System of Musical Composition by Joseph Schillinger

Tonal Harmony by Kostka and Payne

Jazz Composition and Orchestration by William Russo

Composing For Jazz Orchestra by William Russo

Material and Techniques Of Twentieth Century Music by Kostka

The Craft of Musical Composition Vols. I and II by Paul Hindemith

Harmony by Heinrich Schenker

Harmony by Walter Piston

The Orchestra by Ebenezer Prout

Applied Forms by E. Prout

Sonata Forms by Charles Rosen