What Fly Fishing Book?

What do you look for in book about fly fishing such that you will actually
take action and buy it?

In another thread, Larry L. posted what he liked in his 2 favorites out of
140 fly fishing books he owns. He said, "Both greatly affected HOW I think
about fly fishing, fly design, hatch matching, observation of naturals, etc
etc, ... not just WHAT I think. The "what" in both books is largely out of
date, but the "how" is what makes "new" possible."

What kind of fly fishing book is YOUR favorite and why?

John