I've read a lot of Gaiman's Sandman series, which I adore, but this was my first of his novels. Neverwhere is set in London. London Above and London Below. The story follows Richard as he saves a bleeding girl from the attentions of two nefarious gentlemen, an act which transforms his life. He falls through the cracks of London (Above) and descends into the dark, rat-infested sewers of London Below. Here he must help the girl again in order to try and find a way back to London Above and his old life.
Gaiman says in the intro that he wanted to give adults the opportunity to emmerse themselves in the kind of world that is the usual fare of children's stories. And he has absolutely done this. Neverwhere is brilliant and by turns dark and sinister and very funny. I would add though that but for a few mild bits of effing and jeffing and some moderate sex descriptions there isn't much here that would trouble a middle grade reader (10-12 YO). Richard is a very childlike character, as is Door, the other hero of the story. But these are not criticisms, merely observations. If you like NGs work, you'll love this I'm sure.