Neverwhere - Neil Gaiman I've read this book so many times and it never fails to impress.
Richard Mayhew heads down to London to start a new job. He meets a girl, they get engaged, and then...he "falls through the cracks" and his life isn't the same.

There are a whole lot of people like Richard, who for whatever reason have stopped being noticed. They congregate in London Below, with its tunnels, sewers and Tube stations. But there's more to it, as station names mean more than a place on a map.

Gaiman makes a completely different London, with a bizarre mix of characters and places that despite their differences give a sense of belonging in this strange but familiar place.

The characters are all well done, even knowing the ending doesn't detract from reading it again.