Mr. Coté publishes the Double Gun Journal, a quarterly publication of coffee table quality, that illustrates fine, and otherwise, double guns from every corner of the world. He has done so since about 1989, I believe. Furthermore, he and several partners have published a two volume history on Parker guns, any student of same (not me, by the way, I'm afraid) should consider manditory reading, if not owning, but, owning would be far better.
The world has come a long way since Peter Johnson's tome, by the way. You are quite far behind the curve, which, fortunately, is correctable.
Best,
Ted