If you could only have one book on fly tying; what would it be?
In my opinion there is only one book to have and that is The Flytyer's Benchside Reference to Techniques and Dressing Styles. There are no patterns in this great work but practically any technique or method you could ever need to execute is in there. Mind you this not a beginners book but once you have the basics this is really the only book you'll ever need. Not exactly cheap either but worth every penny.
I personally have not much use for most books that are out there; they are mostly pattern books. Mostly what I have is a few books on classic salmonfly tying and history on flies. My most prized is The Practical Angler by W.C. Stewart the 1907 Centenary edition.