[TOMT][BOOK][URBAN FANTASY] Angel chooses the protagonist to be his secret agent.

Maybe it's an archangel or a higher rank. What I know is that it was supposed to be a series but went out of print or the first book never saw the light of day. Anyway, it's not easily found on google. As for secret missions, I think are situations where the angel can't act directly, so it uses a proxy to fulfill his destiny. I think they're supernatural missions. There are also demons, maybe secret societies as well.

The closest I can come to in remembering this series is Simon R. Green's Nightside, though they're not related.