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Beginning with the basics of music theory
By an Amazon.com customer
Tonal Harmony, With an Introduction to Twentieth-Century MusicTonal Harmony, With an Introduction to Twentieth-Century Music by Stefan Kostka
Buy used from: $17.00
The most practical jumping-off point for tonal theory
Form in Tonal Music: An Introduction to AnalysisForm in Tonal Music: An Introduction to Analysis by Douglass Green
Buy new: $106.66 / Used from: $35.00
Everything you wanted to know about form and are afraid to ask
Study of Counterpoint: From Johann Joseph Fux's Gradus Ad ParnassumStudy of Counterpoint: From Johann Joseph Fux's Gradus Ad Parnassum by John J. Fux
Buy new: $12.21 / Used from: $7.00
The simplest and most fulfilling approach to understanding counterpoint
Technique of OrchestrationTechnique of Orchestration by Kent Wheeler Kennan
Buy used from: $2.38
By far the most comprehensive instrumentation and orchestration guide
New Sounds for WoodwindNew Sounds for Woodwind by Bruno Bartolozzi
Buy used from: $75.00
Serves as an excellent referral for employment of extended woodwind techniques
Technique of My Musical LanguageTechnique of My Musical Language by Oliver Messiaen
A unique milestone in explanation of process by any composer
The Structure of Atonal MusicThe Structure of Atonal Music by Allen Forte
Buy new: $27.00 / Used from: $19.99
If you can get through the "math" in this, doors will open for new compositional directions
Twentieth-Century Music: A History of Musical Style in Modern Europe and America (The Norton Introduction to Music History)Twentieth-Century Music: A History of Musical Style in Modern Europe and America (The Norton Introduction to Music History) by Robert P. Morgan
Buy new: $52.50 / Used from: $23.00
Morgan's treatise is one of the better historiographies/analyses I have encountered
Twelve-Tone Tonality, Second editionTwelve-Tone Tonality, Second edition by George Perle
Buy new: $60.00 / Used from: $42.50
Perle's explanation of serialism technique outweighs many other offerings
Gravity's Rainbow (Classic, 20th-Century, Penguin)Gravity's Rainbow (Classic, 20th-Century, Penguin) by Thomas Pynchon
Buy new: $12.92 / Used from: $3.72
As a last resort for "original" ideas, take a hint from Mr. Pynchon.