I'd also reccommend Walter Piston's "Orchestration". I have Forysth, Rimsky-Korsakov, Adler, Piston, and there's one more I can't recall off of the top of my head at this point, but I find Adler's the most helpful. Granted, I have the second edition so I don't know what, if anything, has changed in the newest edition.
Frankly, I wouldn't settle on any one book, as books on orchestration are just like sample libraries -- not one is going to be enough and there's always something missing in one that is in another. If I had to suggest one, it would be Adler's book, and then go for Piston's. Those are the two that have been most helpful to me, but if I have to check a fact or situation I usually find myself reaching for Adler's first.
Frankly, I wouldn't settle on any one book, as books on orchestration are just like sample libraries -- not one is going to be enough and there's always something missing in one that is in another. If I had to suggest one, it would be Adler's book, and then go for Piston's. Those are the two that have been most helpful to me, but if I have to check a fact or situation I usually find myself reaching for Adler's first.