Шестой Дозор by Sergei Lukyanenko

An experienced Night Watch operative, Anton Gorodetsky, becomes entangled in a dangerous conspiracy that threatens the fragile balance between Light and Dark Others; as he investigates mysterious crimes and betrayals, he uncovers hidden agendas within both Watches, confronts painful personal secrets, and is forced into moral choices that pit duty, love, and the precarious peace keeping the supernatural world from spilling into ordinary life.