Deadly Game (GhostWalkers, #5)

It begins as a mission to protect a politician from an assassination threat. But the operation takes an unexpected turn when Mari, a mysteriously beautiful GhostWalker, is taken hostage. At the same time, Ken Norton, expert assassin and himself a GhostWalker warrior, is on a mission of his own—one that reaches into Mari’s own past. No stranger to the ways of violent warfare, Mari must join forces with Ken and trust his every move—each one more intimate than the last.*Publishers Weekly