"The Dry Fly: New Angles" is an excellent book that a well researched and thought out approach to dry fly fishing, particularly with regards to choosing attractor patterns. It complements many of the other great books on fishing dry flies.
The Guide to Prospecting Trout steams is also an excellent book that focuses on fishing non-hatch periods and covers the types of conditions where you may want to try dries vs nymphs vs wets vs streamers, etc. This book is one of the few that I usually carry with me on trips (in my luggage, not on the water).