The Enigmatologist by Ben Adams

Twenty-something, John Abernathy is disillusioned. His job as a private investigator is unfulfilling, and he can't find work in his chosen field, Enigmatology, the study and design of puzzles. He is about to quit when the National Enquirer calls. A woman in Las Vegas, New Mexico sent them a photo of someone who's supposed to have died 35 years ago - Elvis Aaron Presley. And they need John to investigate it. When the Elvis impersonator, Al Leadbelly, is murdered, John investigates, finding Air Force colonel, Alvin Hollister - convinced Leadbelly has information regarding Elvis's death - at the crime scene conducting his own investigation. John discovers his great-great-great-grandfather's journal - unearthing a conspiracy entwining Elvis, shape-shifting aliens, and Mary Todd Lincoln. Now John must solve Leadbelly's murder and discover if he really was Elvis, an obsessed fan, or something more.