That book on .410's only, from a few yrs ago, was a great read on the unusual niche of our sport. "American and British .410 Shotguns" , Ronald S. Gabriel.
Walt Snyder's book is fascinating.
SDH's book on fine double shotguns was really an insight to the craft without being overly technical. I haven't seen his new books. But, I'm sure they're equally interesting.
For a little book, Shotgun Technicana, McIntosh or Gunfitting, Michael Yardley are both nice books to have for the guy that dabbles in shotguns.