Do you want an FF "book" or do you want a novel?
If the former, any of the suggestions already posted are good ones.
The Vanishing Trout offers fascinating history. The Landis book, despite the glut of similar works out there for virtually every state/region of the country, is unique in its accuracy and focus on fisheries quality and nuts and bolts access information and is head and shoulders above the pack as an informative guidebook. Anything Lyons wrote is worthwhile as a great introduction to the particular wealth of essay writing out there on fly fishing. I think, when we look at sports-related prose, only baseball has anywhere near the same amount of fine literature out there to choose from.
But if you want a "novel", I mean an actual novel, as good and as revered as Maclean's "A River Runs Through it" may be (actually a novella), the best novel out there that I've found with fly fishing as a central theme is Joseph Heywood's "The Snowfly". It isn't completely about fishing, but fishing is a central anchor of the story and plot. "The Snowfly isn't just a fine fishing novel. It is a fine novel period, irrespective of genre.
But I've said all this in previous threads on the same topic. Worth repeating though, IMO.
http://www.amazon.com/Snowfly-Novel-Joseph-Heywood/dp/1592285104