Gierach's "Standing in a River Waving a Stick" is my favorite of his, but you really can't go wrong with his books. They're not a complicated read, but he knows his stuff, and he knows why he enjoys fly fishing. He also knows how to write about it in a way that would make you kill to get on the river while reading his essays.
"A River Runs Through it" is good read too. It's a shorter book, which can be knocked out in a day if you choose to spend the time on it. Its purist fly fishing to the core, this book makes more of comment on life in general than just simply fly fishing.
"The Old Man and the Sea" by Hemmingway isn't about fly fishing, but its one of my favorite books of any genre. I read this book once or twice every summer, and definately when I'm at the beach. It's only 90 pages, and I highly recommend everyone to read it.
I'm about to start "A Pale Morning Dun" by Jeff Hull. I'll let you know how it is.
"I've got to stop wishin, I've got to go fishin"