I recently dug up a book at school, by Cecil Forsyth. It was written in 1914, so it's a little outdated, and some of his scientific explanations about how woodwind instruments work are a bit dubious. But it's written very well, and is certainly informative. I found his chapters on horns and strings to be especially useful. There's also alot of interesting, if slightly useless, information about the evolution of each instrument. And, best of all, there is an abundance of examples from Wagner!
I also quite enjoy his slightly sarcastic tone. He keeps making snide comments about instruments and composers, and he has a right go at "the German school of oboe playing" :D And, rather than "tuned" and "untuned" percussion, he uses the words "musical" and "unmusical". That made me chuckle!