Some websites that may help:
www.shotgunreport.com
Check out the technoid, he has a lot of information on chokes, shot patterns, loads,dram equivalents and other inconsequential drivel.
www.gunshop.com
This site deals mostly with double shotguns, but there is a wealth of information on shotguns if you will dig for it.
www.shootingsportsman.com
Great magazine for wingshooters, the only mag I do not throw away. Beautifully illustrated and has some of the best game recipes you will find.
www.shotgunsportsmagazine.com
Mostly trapshooting but also has other shotgunning articles.
www.shotgunsports.com
Shotgunners BBS. You will find shotgunners to be a yawn compared to TFL. but the site may have usefull information.
Also look for Double Gun Journal in your local Borders or Barnes & Noble. This quarterly journal(Too classy to call it a magazine) has a wealth of information about shotguns. A warning, it ain't cheap, and don't even look at the prices of the guns for sale inside.
Two books;
Clay Target Handbook by Jerry Meyer
Taking More Birds by Dan Carlisle
That should about cover it!
Geoff Ross