On two ends of the price spectrum are Gun Craft by Venters, and Shotgun Technicana. There's a book written from the amatuer Smith's point of view by a fellow that used to visit here, Tony Treadwell, called Vintage English Shotguns and Their Restoration.
There're countless tidbits sprinkled in books dedicated to makers and shooting. Libraries can get a little excessive, but in a good way. Don't rule out best rifle making references. Another good online source to look around on is the Explora on the Westley Richards site, if your weren't aware of it. Best of luck with your search.
