One Gun Book publisher that is still very much alive is Wolfe Publishing of Prescott, Arizona which also produces "Rifle" and "Handloader" magazines. I have bought numerous gun and hunting books from them including one on Double Rifles, but I cannot recall any double shotgun books published by them. They obviously do lean toward the single projectile, but a budding writer might keep them in mind for a prospective publisher.

Voting for anti-gun Democrats is dumber than giving treats to a dog that shits on a Persian Rug