Textures and Timbres by Henry Brant

A life’s work in codifying the principles of orchestral texture. In some ways, it’s a very narrow view into one specific element of orchestration, and it also is a somewhat personal set of guidelines for Brant’s own profound compositions. And yet it’s a good sourcebook for composers playing with ideas, and wanting to understand timbral relationships. Highly recommended, after you’ve done quite a bit of other foundational reading.