"Stillwater Fly-Fishing for Trout", by Chris James, cos he's clear-thinking, lucid, has strong opinions well explained, and doesn't like wasting money! He cuts through a whole load of "crepe" and the media establishment probably hate him.
If you find it on ebay or in the remainder shops, grab "Techniques of trout fishing and fly tying" by George Harvey. This is another independent thinker and fine teacher at work; it may refer to American waters, but it's the way he thinks that makes this a great book.