Nick Ryan Series by Reed Farrel Coleman

Every cop in the city knows his name, but no one says it out loud.In fact, they don't talk about him at all. He doesn't wear a uniform, but he is the most powerful cop in New York. Nick Ryan can find a criminal who's vanished. Or he can make a key witness disappear. He has cars, safe houses, money, and weapons hidden all over the city.With conflicted loyalties and a divided soul, he's a veteran cop still fighting his own private war. He's a soldier of the streets with his own personal code.