The Innocent by David Baldacci

The Innocent is the first novel in David Baldacci's Will Robie series, a thriller about a government assassin who goes rogue after a mission in Washington D.C. goes wrong, forcing him to protect a teenage runaway whose parents were murdered, uncovering a vast conspiracy that puts both their lives in danger. The book follows Will Robie as he becomes a target himself and teams up with the girl, Julie Getty, to find the truth behind her parents' deaths and escape his own government.