Midnight Never Come by Marie Brennan is a rock-solid, highly entertaining tale of intrigue, magic, and adventure. It received a starred review in Publishers Weekly, which read in part: "Stunningly conceived and exquisitely achieved, this rich historical fantasy portrays the Elizabethan court 30 years into the reign of the Virgin Queen, often called Gloriana. Far below ground, her dark counterpart, heartless Invidiana, rules England's fae. Brennan pairs handsome young courtier Michael Deven, an aspiring agent under spymaster Sir Francis Walsingham, with bewitching fae Lune, who attempts to avoid Invidiana's wrath by infiltrating Walsingham's network in mortal guise. History and fantasy blend seamlessly as Deven and Lune tread their precarious tightropes between loyalty and betrayal." This novel is definitely worth seeking out.
I interviewed Brennan via email recently. For more information on Brennan and the novel, check out the official website for the novel (they're running a contest with a $500 prize).
Amazon.com: Why is underground London so compelling?
Marie Brennan: Underground things of any stripe are neat because they're hidden--the original meaning of "occult." The notion of there being a secret world right next to the ordinary one produces an interesting frisson for the reader or writer--that's the appeal of urban fantasy, at least for me--and if it's underground, it's dark, it's buried, it's a good place to find creepy things or relics of the past.
It isn't just London, of course; Ekaterina Sedia, for example, does it to Moscow in her debut novel. London works well, though, because it's familiar enough to be recognized by your average Anglophone reader. You need a city old enough to have a buried history--I don't think it would work in a suburban housing development, though I suppose you could try--and you need your reader not to feel totally lost. As much as I'd love to read a fantasy about, say, underground Hong Kong, I know nothing about the city or its history, and so much of what's cool in such a book would be lost on me.