There are quite a few books on orchestration..
There is a book by
Berlioz ("
Treatise on Orchestration", I think), revised later by Busoni (if I remember correctly). I have absolutely no idea whether it is good or not, as I haven't read anything from it, but I read in the Adler book of orchestration that Berlioz says the Cello is not as agile or capable of virtuosity as the violin and viola, which is not generally true, so it might be a bit outdated.
There is also the book called "Principles of Orchestration" (it can be found online
here, with audio examples, I think) by
Rimsky-Korsakov. Although it is generally considered to be a good book, he only uses examples from his own works, which makes the book a bit arrogant and not too representative of the musical literature.
If you're looking for more recent stuff, there's a book called "
Study of Orchestration" by
Samuel Adler (b. 1928), which is currently in its 3rd edition and comes with (optionally) 6 CDs with all the examples of the book recorded by real musicians. There is also a workbook available, with orchestration exercises. The Adler book is generally considered a really good book, it has more than 500 examples from all over the place (from Bach to Mozart to Brahms to Bartok to Schoenberg to Crumb to anything really), and it mentions a lot of physical practicalities (like how the instruments are played and what limitations this imposes on the abilities of the instrument), various techniques, extended techniques and effects, orchestrating for a family of instruments or for combinations of families (As well as a bit on parts), and also has a small "extra resources" list at the end of each instrument or important chapter, with many works that exhibit a good use of the instrument/technique/orchestration/whatever. It also has a huuuuge bibliography list, which might prove worthy if you're interested in searching for a particular subject (say, extended techniques on the flute, or multiphonics on the saxophone).
However, the Adler book is lacking a lot in that there are many instruments it doesn't mention, like the electric guitar, harmonium, accordion, voices etc.
So, here comes the
Blatter book, which as far as I know is very very good. It is as good as the Adler, with extended techniques and orchestration techniques (and examples), and includes the aforementioned instruments as well as others. I haven't read all of it, but I've read some parts of it when I needed, and it proved to be very helpful.
I also know that
Walter Piston has a book on orchestration and
Gardner Read (who has written many good books on notation) has written a book called "
Thesaurus of Orchestral Devices", but I haven't read either of the books, or read anything from them, so I can't really say anything about them. There is also an online
guide to orchestration by Alan Belkin.
There is also a small list of famous instrumentation books here:
Orchestration - Wikipedia, the free encyclopedia but I assume you've already been there, if you've searched for orchestration on the net
So, there you go, my opinion on orchestration books.