Trey Combs is already mentioned by SalterLee, but as his
Steelhead Fishing and Flies was a great inspiration for me when I started tying flies for our sea run browns (we call them Sea Trout) in the early 80s, as well as inspiring the styling of my first rod builds (on Sage S-glass blanks), I would like to mention him again.
This book includes a chapter on the history of steelheading in the PNW and historical/local perspectives are frequent in the long chapter on flies and their use.
I have the second printing from 1978.
Highly recommended