Two books I enjoy, and I think I originally heard about them on here, are "Fisher in the Hills" by Robin Ade and "Fishing A Highland Stream" by John Inglis Hall. Another I like (which might be a local/self publication) is "Of Fish and Men - Tales of a Scottish Fisher", particularly as the author writes a lot about his fishing in Caithness & Sutherland.
I'd very much second some of the other suggestions; the "Pursuit of Stillwater Trout is a thought provoking book and, although much of the writing is about the authors experiences on reservoirs, you can apply what's written to pretty much any kind of fishing/fly fishing. "The Trout & The Fly" (Clarke & Goddard) and really captured my imagination when I first read first as a youngster; the underwater photos of flies, leaders and fly lines are worth getting the book for alone. It will also want to make you tie your dry flies upside down

You can't go wrong with Gierach, Rangeley-Wilson (..ain't got DEET, ain't got shit...

) or the Bob Wyatt books.