Book 6 in the Ryker Returns series.
"James Ryker knows the ghosts of his dark and violent past are never far away. Yet he’s still taken aback when he sees Leia Devereaux’s face at his door. Even more surprising is that the woman he knows to be a lethal assassin claims she’s not there to hurt him, but to ask for his help in keeping the stranger accompanying her, David Muller, alive. She says Muller is more valuable than Ryker can imagine. But Ryker isn’t given any time to investigate before chaos descends upon them. In the aftermath, Devereaux disappears and Ryker is left with only questions."