The Hit by David Baldacci

Will Robie is a government hit man. The only other person who comes close to his skill level is fellow government hit person Jessica Reel--but she's gone off and started killing supposedly good guys. Robie is sent out to find her and bring her in dead or alive. But he soon realizes there may be a method to her madness and she may actually be trying to save the country from enemies within its own ranks.

The Hit is the sequel to the equal good The Innocent. I recommend both for good summer reads.