I have posted my paper (which may be edited soon. First draft) about five significant books written by composers concerning either composition in general or their own style.
I think a lot of the material in these books will give valueable insight to composers. The books examined are:
The Schillinger System of Musical Composition by Joseph Schillinger
The Technique of My Musical Language by Olivier Messiaen
The Craft of Musical Composition by Paul Hindemith
Genesis of a Music by Harry Partch
New Musical Resources by Henry Cowell (composer of "The Banshee")
Feel free to peruse this paper and post your thoughts and opinions here. The section on the Schillinger is particularly long, and although I don't pretend to fully understand it, I think I give a fairly in-depth synopsis of it. The Schillinger should be of particular interest to the film composers here. For years his theory of "Semantics" in music was a big influence on film composers.
** Some of the theories in these books can be controversial, let's try to keep the conversation civil, but spirited.