Neverwhere by Neil Gaiman
Between being busy and not really knowing what to say about this novel, I've been pushing off writing about it online. As I said about the last Gaiman book I read, apparently, I'd chosen the wrong books of his to start with since I never got the big deal. I've quite enjoyed the last two novels of his I've read, though.
In Neverwhere, Richard helps a young woman, and as a result, gets sucked into a life in London Below, losing his entire existence in London Above. In London Below, many things can be taken literally, such as Baron's Court actually contains a Baron on its tube. In order to regain his normal life, Richard finds himself on a quest with Door, who is in search of her family's killer. Many of the supporting characters are very entertaining and colorful.
In the afterword/back of the book interview, Gaiman said he's wanted to create a magical city novel about London, and he definitely succeeded. Of course, between Harry Potter and His Dark Materials Trilogy, there are a few other novels with that theme, but those are all children/young adult.