Neverwhere

Neverwhere by Neil Gaiman. mass_market edition. ISBN: 9780380789016.

Richard Mayhew is a plain man with a good heart -- and an ordinary life that is changed forever on a day he stops to help a girl he finds bleeding on a London sidewalk. From that moment forward he is propelled into a world he never dreamed existed -- a dark subculture flourish in abandoned subway stations and sewer tunnels below the city -- a world far stranger and more dangerous than the only one he has ever known...Richard Mayhew is a young businessman with a good heart and a dull job. When he stops one day to help a girl he finds bleeding on a London sidewalk, his life is forever altered, for he finds himself propelled into an alternate reality that exists in a subterranean labyrinth of sewer canals and abandoned subway stations below the city. He has fallen through the cracks of reality and has landed somewhere different, somewhere that is Neverwhere.