I have read this before, but ut was a while ago, and when I read it I rewatched the TV series, which has aged remarkably well, and the radio dramatisation, which is a superb piece of work. the notion of a paralell London below regular old London above seems like the sort of idea that must have been bumping around in the brief few days when Urban Fantasy sort of meant fantasies set in cities, but here comes Neil Gaiman executing what SEEMS like a thoroughly obvious idea, underlining it by taking his cue from literal inventions based around placenames, as if nobody else could ever have thought of it or written it in quite this way. Probably right about that.