Susan Kuhlmann's KNAVE, FOOL, AND GENIUS (1973) is the only source that comes to mind immediately. It has much discussion of Melville's CONFIDENCE MAN and Twain, but I don't recall anything being clearly documented. There's another book that I think discusses this subject, but I can't recall the title. It studies the theme of impostors or con artists in Twain's works (and others) and I think it has the word "impostors" or "imposture" in the title. Kevin Mac Donnell Austin TX