You can purchase Paul's books through Amazon, MidCurrent, and many book stores and fly shops. His Bug Book was featured as Amazon's third highest rated fly fishing book by wideopenspaces.com and the number one entomology book of all time by The Book Authority.
Fly Fishing Guide to the Upper Delaware River (1st and 2nd ed):
Tying Dry Flies:
Pocketguide to Pennsylvania Hatches:
Pocketguide to New York Hatches:
The Bug Book: