Neverwhere (Author's Preferred Text) by Neil Gaiman

Richard Mayhew is a young man with a good heart and an ordinary life, which is changed forever when he stops to help a girl he finds bleeding on a London sidewalk. His small act of kindness propels him into a world he never dreamed existed. There are people who fall through the cracks, and Richard has become one of them. And he must learn to survive in this city of shadows and darkness, monsters and saints, murderers and angels, if he is ever to return to the London that he knew.