Source and training books for composers:

1) theory of music (occidental and non-occidental; european, accademic,
traditional and avant-garde music - obs.:  tribal/folk/pop music too!)
2) history of music
3) notions of Acoustics
4) orchestration and instrumentation (ex::Walter Piston, N.
Rimsky-Korsakov, A.  Casella or H.  Berlioz)
5) musical training (ex.:  Pozzoli, Bona, "Ritmica" - Eduardo Jose Gramani
- Unicamp-Brazil, "Elementary training for musicians" - Hindemith - and
others)
6) harmony (ex.: Hindemith); counterpoint (ex: W. Piston)
7) composition and analisys of musical forms:
A. Schoenberg: "FOUNDAMENTALS OF MUSICAL COMPOSITION"
A. Schoenberg: "MODELS FOR BEGINNERS IN COMPOSITION"
P. Hindemith: "CRAFTS OF MUSICAL COMPOSITION"
P. Hindemith: "PRACTICAL FOR TWO-VOICES COMPOSITION" and thousand of books
8) hearing music with score (if possible)

And a good teacher, experience, and IMAGINATION!!!!

Edson Ortolan
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http://sites.uol.com.br/eortolan