Neverwhere

Under the streets of London lies a world most people could never dream of.
When Richard Mayhew helps a mysterious girl he finds bleeding on the pavement, his boring life changes in an instant. Her name is Door, she's on the run from two assassins in black suits and she comes from London Below.
His act of kindness leads him to a place filled with monsters and angels, a Beast in a labyrinth and an Earl who holds Court in a Tube train.
It is strangely familiar yet utterly bizarre. / Neil Gaiman