I've got a few ideas <g>
The Defensive Shotgun, by Louis Awerbuck, for starters. Mr. Awerbuck also did a pretty good video of the subject as well, availible from Paladin Press & others.
Follow-ups would be The Tactical Shotgun by Gabe Suarez and The Serious Social Shotgun by Steve Mattera, published by (I think) Zeidecker Press, it's a bit harder to find but the better of the two books.
You didn't ask, but I'll throw it out anyway: Training, Awerbuck does an excellant 3 day shotgun class. Bill Jeans, currently of Morrigan Consulting, is traveling to teach classes as well.
I've got copies of some articles should you be interested in them.