This is book number 5 in the Ghostwalker Novels series.
Deadly Game 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.