Why not go back to the Victorian classics? Kelson's "The Salmon Fly" is so full of information on fishing and tying it is a tome that has had a major influence on my steelhead fishing. I have found Kelson to have been much maligned by many; however, I have also found those who disparage his book almost never have read any of it, let alone the whole book. It is a very large and voluminous book; but one that I think any Atlantic Salmon or steelhead fly fisher can learn much from. It is not just a book on tying classic feather wings, it is really a fishing book that is disguised as a book on fly patterns.
Traherne's book "Salmon Fishing with the Fly" is another book that, although not anywhere near as voluminous and complete as Kelson's, has much of value in it. Hill's "Salmon Fishing", which was recommended to me by Alec Jackson, is another one of those wonderful little tomes that is full of wisdom for fishing the low-water of summer and fall, and a book that I am glad I procured.
All of the above titles are available through Fly Fishers Classic Library (very high quality bindings, inks, and paper) and they are located in the UK.