The Second Door — Christopher D. Abbott

At the end of his own birthday celebration, Major Peterson shocks his seven guests by leaving the breakfast table, going into his study, and apparently shooting himself...Inspector Delaware discovers conflicting evidence, however, and calls for help from Sherlock Holmes. Upon their arrival, Holmes tells Watson he believes there's more to Peterson's death than the evidence might suggest. Still, the facts continue to show that Peterson committed suicide-until they discover something that changes everything...