As the first orchestration book I know of to have CDs of the musical examples available, Adler is definitely worth acquiring if you've a hundred quid or so to spare. I find the text a bit of a let down though. First, it's an American book, and many American instruments differ from European ones, a trap for the European reader. Secondly, there's a knack for writing idiomatic stuff for all instruments which sometimes deserts even the greatest composers (Brahms is not the greatest writer for strings, for example), and Adler doesn't really get inside the instrumental character in the way Piston's book does, or better still Cecil Forsyth's "Orchestration", written nearly a hundred years ago. Infuriatingly, Adler's book is badly laid out so if you want to leaf quickly through to find, say, the bottom note of the piccolo there are no headers telling you which instrument is on which page. If I had to use just one book it would be Piston's, because it's succinct, clear and much better written than Adler. I don't know Blatter, and I've never seen Berlioz's book, or Rimsky's. But the Edwardian gem is Forsyth - detailed, funny and unafraid to trash bad orchestration even from the greats.