The Hit - David Baldacci, Pan Books

The trap is set. Failure is not an option. When government hit man Will Robie is given his next target he knows he's about to embark on his toughest mission yet. He is tasked with killing one of their own, following evidence to suggest that fellow assassin Jessica Reel has been turned. She's leaving a trail of death in her wake including her handler. The trap is set. To send a killer to catch a killer. But what happens when you can't trust those who have access to the nation's most secret intelligence?