Target Lancer by Max Allan Collins
Long before November 22, 1963, Nathan Heller, "P.I. to the Stars," knows that a conspiracy is in the works. Several years earlier, Heller had been involved with the Kennedys, the Mob, and the CIA in the early stages of a plan to assassinate Fidel Castro. Shortly after, Heller's Mafia contact is murdered. After being interrogated by gangsters and contacted by US Attorney General Robert Kennedy, Heller realizes that he may be the one person who can prevent a devastating political assassination. Only he knows all the players; only he knows why a web of conspirators has targeted the man known to the Secret Service as "Lancer," John Fitzgerald Kennedy.