I don't think there is really a book on black powder double rifles. A full set of the DGJ will certainly provide a number of good articles. The Big Bore Rifle by McIntosh has a section or so as well as Shooting the Britsh Double Rifle. Books on Holland , Purdey and Greener will have some info on their guns. I can't think of a book solely devoted to pre 1900 underlever guns, if someone knows of one I would like to buy a copy myself.