"Many persons have such a horror of being taken in," wrote P. T. Barnum, "that they believe themselves to be a sham and are continually humbugging themselves." Mark Twain enjoyed trading on that horror, as the many confidence men, assumed identities, and disguised characters in his fiction attest. In Dark Twins, Susan Gillman challenges the widely held assumption that Twain's concern with identity is purely biographical and argues that what has been...