1) Features angel(s)
2) Is not overtly religious or "inspirational" (e.g., I don't want so-called Christian fiction, or an adventure about dangerous Biblical artifacts and an OMG!Church Conspiracy!)
3) Is set in more or less modern times
4) Is set in the real world, or at least close to it (i.e., not a fantasy realm)
5) Features some teenage main characters, though doesn't have to be aimed entirely at young readers
Yes, I've done internet searches, and my eyes are getting tired of sorting through the slush. The only book I can think of that comes close is Good Omens, but that's still not very similar to my own. For one thing, it's a bit more frivolous (which of course is one of its attractions); and for another, it's ever so British (which is also one of its attractions, but I digress).
In the movie world, Michael and City of Angels/Wings of Desire are the closest, but they don't deal with teenagers.
Searches for teen angel fiction come up with lots of Buffy novels about the character Angel, who, despite his name, is a vampire and thus no help at all. So I'm giving up and asking youse-all.