Given that I have a rather large and complete library of shotgunning books I was taken aback at a gun show recently with a "new" book about actually making a SxS from scratch. And while I don't plan to scavange barrel tubes and make lock parts or carve out a stock, the book proved facisinating reading and brought me up to speed on what every good gun mechanic ought to know.
Fact is that a good percentage of the questions posited on this and other double gun websites are remedial and repetitive, showing that many participants do not have a basic understanding of what a thunder stick is or how one is put together. For those who simply shoot their Parkers and Purdeys, speaking the language and understanding manufacturing techniques isn't important, but at the level of inquiry I detect on this website I recommend: Recreating the Double Barrel Muzzle Loading Shotgun by William R. Brockway (1985, 2003 2nd ed.) George Shumway Publisher, Inc. 3900 Deep Run Lane, York, PA 17402
I have no interest in this book beyond the content, which I find valuable because everything is covered from proofing barrels to making breech blocks to building locks from scratch, case hardening, wood finish, stocking, etc. And while it presumes a muzzle loader will be built from scronged parts or from scratch, the latter-day details of making a breechloader are easily visualized (in the context of difficulty). Two other good books that aid in understanding the basics are:
Fine Gunmaking: Double Shotguns by Steven Dodd Hughes (1998--this hard cover book has been remaindered out by Krause, but is readily available on the Internet).
The American Shotgun by David F. Butler (1973) available on the Internet, both hard- and soft-cover.
The three good books cited above, when read and digested, should make anyone a certified entry-level "Gun Crank." EDM