As already mentioned, this is a solid, mostly readable guide to the instruments of the orchestra. It is of course important to bear in mind that this book was written just before the first world war, and the standard orchestra has changed a little. This means that we get a thorough treatment of some totally obsolete instruments I'd never heard of. (Some are talked about as being obsolete then, in 1913, so they are really really obsolete now, Nevertheless its a nice history of the evolution of instruments).
One final point that should be emphasied - this is really an instrumentation manual, not orchestration. It will tell you how to write for a specific instruments, their capabilites, ranges, strengths and weaknesses, and so on. It won't tell you very much about combinations of instruments, ways of scoring passages, etc - this is more the domain of Rimsky-Korsakov's Principles of Orchestration or one of the more hefty Orchestration texts.