The best way to understand orchestration is through experience and interaction with the material. Consider score-studying the parts of specific instruments, or a concerto written for the instrument. While manuals are comprehensive, they do not provide the logical flow of empirical learning that one would get from actually learning the instrument. Knowledge is different than practice.
With that said, here is a space where I will compile free sources on the webs for those who are interested in learning orchestration and instrumentation.
Sources will be from certain public domain libraries, articles, etc.
Anatomy of the Orchestra by Norman Del Mar (Google Books)
Principles of Orchestration by Rimsky-Korsakov (Gbooks)