You can purchase Paul's books through Amazon, Barnes & Noble, MidCurrent, and many other book stores and fly shops.
Paul's Books at Barnes & Noble
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:
Dry Fly Strategies: (coming July 2021)