Apocalypse Rising Series by Mychael Black

Andrew Blackwell is only one of countless humans who managed to survive post-World War III. Living in the shell of what was once New York City, he focuses on working hard and playing hard. Nothing else matters. But fate has other ideas, and they arrive in the form of a rather surly angel who's been sent to find the one special human who can possibly help save humankind. Thrown into yet another war, Andrew has no choice but to trust Mikhail ... no matter where the angel may lead him.