I recently discovered this wonderful set of ebooks by Montreal composer and professor Alan Belkin. He’s got books on musical form, counterpoint, harmony and orchestration, as well as several other articles and essays. I’ve spent the most time with the orchestration book (since I’m currently orchestrating a symphonic piece for a March performance), and it’s excellent. I found it to be simple, straightforward, and full of great advice. Some of it is fairly common-sense and basic, but I never mind being reminded of the fundamentals, especially for something as complex as orchestration. The other books seem similarly well-written and instructional.
Incidentally, Belkin also authored the annotations to the online version of Rimsky-Korsakov’s Principles of Orchestration, available at NorthernSounds.com.
All of Belkin’s writings are available at his website.