Trout Bum is as good as any to buy. I read Geirach's books over and over.
This may seem a little out of place here.... I have a "how to" book called Fly Fishing the Rocky Mountain Backcountry by Rich Ostoff that I got last year from a used book store. I can't recommend this to everyone, but if you have a thing for the Rockies and really long hikes for unspoiled fishing are your type of thing, I highly recommend it.