Neverwhere

Richard Mayhew was a very ordinary young man. One day he stopped to help a young girl who mysteriously appeared in front of him, lying injured and bleeding on the street. He took her home, cleaned her up and he was more surprised when she told him her name was "Door".
Door asks Richard to find a man called "The Marquis De Carabas", a man who she says can help her. Richard agreed and brought the Marquis back home to see Door. Suddenly, they both vanished in front of his eyes and he thought he was going crazy.
After meeting the two of them, Richard's life turned upside down...taxis won't stop for him, his fiancee has forgotten who he is, he has become invisible to everything and everyone he knew...
The only thing he could do now if he wants to get his normal life back is to go find Door and the Marquis back. The only way he could find them is go back to the place where he first founded the Marquis, under London, the dangerous place which is NEVERWHERE...

This is another book by Neil Gaiman. It's also a good book written by him but this one is not as good as "Stardust". But this one is thicker, and has smaller words.