'Neverwhere' by Neil Gaiman (New York, Avon Books, 1997, ISBN 0-380-97363-4)
This book is great mix between nice and evil fantasy. The story in general tends toward the nice side, but there are some real nasty elements which makes the story very interesting.
The location is London and a man (Richard Mayhew) becomes, through helping a strange girl named Door, part of a total different London: 'London Below'. London Below is an ancient world in the sewerage of London. Together with this girl and others (Hunter, and Maraquis de Carabas) he forms a group of travelers. Richard is in search of the exit of this state he is in because even when he is in London Above nobody can quite see him. Together they find themselves in the middle of manipulations and setups of a certain person...
Every character in this book is excellent, the plot is great and after reading Neverwhere, you will never again walks past the shadowy places of the modern world with the same foolish confidence.
It really is a work of imaginative virtuosity!