A shotgunnner's most useful tool is a well worn library card. Under Dewey Decimal sytem #799 are the books on shotguns and their uses, along with other gun and fishing books.
Some good ones include:
Shotgunning, the Art and the Science, by Bob Brister.
Bruce Bowlen's guide to upland hunting.
Gene Hill's Shotgunner's Notebook.
Darn near anything by Steve Smith.
Same with Mike McIntosh, but the man never met a shotgun with a 5 figure price tag he didn't like.
All in all tho, a good read with plenty of info.
Same for Don Zutz. A bit more nuts and bolts oriented.
Keith on Shotguns. While a bit dated, and full of opinions(as Keith was), it had some tidbits on usage not found elsewhere.
The best resource can be found on ranges. Old shotgunners tend to be a great bunch, and very helpful.
Good periodicals include Shooting Sportsman, Shotgun Sports, and for the "Serious" side of shotgunning, SWAT mag, associated with this website.
Ayoob, Farnham, Suarez,etc, are good sources for "Serious' shotgunning. Chuck Taylor has a good one on shotguns and sub machine guns, but it's hard to find.
Finally, there's two Archives that have scads of good stuff.That's here and over on shotgunreport.com. Read them both for breadth and depth...
HTH...