My top five have already been mentioned.
Pale Morning Done- Hull
The Snowfly- Heywood
River Teeth - Duncan
The River Why- Duncan
Fly Fishing Through a Mid-Life Crisis - Raines

I would also recommend a couple of authors that are very close to my backyard.
Anything by Jim Harrison and Robert Traver (John Voelker)

Be careful, before you know it, you'll have a library full of fish that will keep your mind stirring well through the night into the early morn.