Well, I had Landis' book for a long time, but I don't consider it a fly fishing book. I saw it first in 2000 or 2001, and I was transitioning to fly fishing. So I used it for both bait and fly fishing, and all my family used it as well, for solely bait fishing. It's a trout stream guide, and useful for trout fishermen regardless of tackle choices.
I think I gave that old beat up copy to my dad and bought a new one 3 or 4 years ago, which was well after becoming a FF. The new one rarely travels on trips with me, but it still reserves a spot on my nightstand. I got it signed at the VF show this last weekend, which was pretty cool, but I was a little embarrassed at how good of shape it was in!
But yeah, of fishing books, it ranks up there.
My first dedicated FF book was Joe Humphreys' Trout Tactics. I've bought a number since, but I'd still rank it #1, it taught me how to fly fish. Got that one signed last weekend too, and Joe even added a drawing of a fly!