American Angler Guide to Fly Fishing for Trout: Proven Skills, Techniques, and Tactics from the Pros

This is a decent intermediate guide, with some great insights on nymphing.

This is billed as a book for beginning fly fishers, so if you already know how, buy this book for your friend or children just getting started in fly fishing...but read it yourself first...you won't want to give it away. AJ gives clear instructions on many different and proven techniques for fly fishing, based on field experience. You won't regret buying this book.