Author and title have gone out of my head- naturally: my brain won't hold names any more. Worse, it seems to have gone out of the library search engine's memory. I've gone through the entire alphabet, with the webpage giving me suggestions for stuff I looked up in March, but no luck with Secret Agents in Fae. I've also gone through all my LJ/ DW reading pages, and tumblr as well, but no joy.
I think it has a black and gold cover. Next stop, Bakka Phoenix. But I shall feel like a prat asking for 'a book about secret agents in Fairyland with a black and gold cover' and not buying it if they actually manage to identify it.