Star Trek: Errand of Vengeance, Book One: The Edge of the Sword

Lt. Jon Anderson is the newest security officer aboard the U.S.S Enterprise. He joins the crew at a time of mounting tension between the Federation and the Klingon Empire. Previous battles have been inconclusive, but Starfleet Command fears that the Klingons are gearing up for a major offensive. The Enterprise would be on the front lines of an all-out interstellar war.
But Lieutenant Anderson has a secret: he is actually a Klingon infiltrator assigned to sabotage Starfleet from within. His first mission: kill Capt. James T. Kirk.