I was a bit wrong-footed by this story. Having read the Bookman by the same author I was expecting more of the same and it took me a while to understand what was going on.
I was a little disappointed that there was no explicit explanation of exactly where these angels came from, and why, but for most of it the pace was so fast there was no time to think about that sort of thing.
The hero is actually a heroine, which makes a change for adventure-based stories, and at such a reasonable price I can live with the lack of background detail, which is possibly being left vague to leave the door open for sequels.