Hi Lopat,
Some of the books I would suggest are no longer in print. This means looking in used book stores online. Here's a few:
The Leisenring book mentioned above
The Hidy book mentioned above
Masters of the Nymph, edited by Wright & Migel
Any or all of the Nemes books on Soft-hackles
Fishing the Dry Fly As A Living Insect by Wright
Presenting The Fly by Lefty Kreh
Selective Trout by Swisher and Richards
Trout Hunting, The Pursuit of Happiness by Bob Wyatt
Wet Flies by Dave Hughes
Tying and Fishing Soft-Hackled Nymphs by Allen McGee
These are just a beginning, and others should add more for sure.
Mark