"Trout Streams of Pennsylvania: An Angler's Guide" by Dwight Landis (Troutbert), not a how-to but one of the most valuable resources a PA trout angler can have. I have at least 1 dog-eared copy in my gear bag at all times. And if you're lucky enough to leave it on top of your vehicle streamside and lose it pulling out only to have the author find it, he might return it to you autographed.
Meck's "Pocketguide to Pennsylvania Hatches" and Tom Gilmore's "Flyfishers Guide to Pennsylvania" are two other great books that live in my gear bag and make every trip to a PA stream with me.
I don't own a copy but Dave Hughes "Reading Trout Water" is probably one you want to have especially if you're new to flyfishing or trout fishing in general.
Hunt, Fish, Vote! I support AR and Sunday hunting. Life Member of NRA and TU.
I'll 2nd the McLean's Fishing Encyclopedia. My folks bought me one for Christmas way back and it still gets heavy use ! While not limited to Fly Fishing there is a good section devoted to it as well a Fly Tying(patterns included). Plus a wealth of other "fish knowledge" that will make you a better fisherman.
“Would it save you a lot of time if I just gave up and went mad now?” Douglas Adams