Four Centuries of Gunmaking by Claude Gaier, good overview not all that great on individual makers, books on the Brit trade spoil you. (English edition exist, I have one)
the Art of Gun Engraving by Claude Gaier (English edition exist, I have one)
Both can be found through ABEBooks.com
I imagine there are some better books on Individual gunmakers, but they must be in French and not accessible if you cant read it, much less find them
Reference the little gun website, a good deal of good data, the books from it are basically reprints of the website.
Another good place to look is the Russian shotgun collectors website. Some articles are in English and those that are not can be web translated enough to be of some use.
https://shotguncollector.com/articles-in-english/http://www.hunting-heritage.com/blog/index.php/2018/04/30/jules-bury-liege-best-gunmaker/