As I read I kept picking out areas where I thought I sensed other literary influences: there were shades of Alice in Wonderland, The Wizard of Oz, Paradise Lost, &c. &c. (alright, in the case of the second title the reference was more cheekily direct, and extremely well played). It's a darkly entertaining story, but also something of a grim satire on the state of human society and the plight of those who fall through the cracks.
If you've ever wondered about what strange worlds exist below the subway platforms (and, having lived in a fairly large city for going on five years now, I know I certainly have), and if you can let your imagination run wild a little bit when you read (a skill I am infinitely thankful for), I suspect you'll really enjoy Gaiman's exquisite novel. It's a treat.