Neil Gaiman

Neverwhere

A Londoner helps a girl in trouble and afterwards finds that he has ceased to be a person, nobody notices him. He finds an alternative magic 'London Below' under London itself to which he apparently now belongs. This world is populated by people who have fallen through the system, outcasts. He now has to find a way to again become a 'real' person in London Above.

This is not a very original story, but it does have a lot of charm and quite a few funny characters and a good dialogue to go with it. It is very much a book about London and if you've been there then that's almost reason enough to read Neverwhere.