Broken Wolf - Moonbound Wolves, #6

Paying a debt may cost him his life-or his heart. When an old friend seeks his help, Russian mercenary Vadik Nabatov doesn't hesitate for a second. After all, he owes Luther his life. But something is off. Luther isn't the same, and even a professional like Vadik might be in over his head. The last thing he needs is to enter into enemy territory with Andrea VonBrandt, who stirs up a desire in him that he thought was long gone. But sitting back and letting Andrea go into a madman's lair as his submissive is asking too much.
She can't possibly face a crazy billionaire alone. Now, it's up to Vadik to save the woman, so he can save his heart.