Neverwhere by Neil Gaiman

(from the jacket)

Richard Mayhew is an unassuming young businessman living in London, with a dull job and a pretty but demanding fiancee. Then one night he stumbles across a girl bleeding on the sidewalk.

He stops to help her--
and the life he knows vanishes like smoke.

This is an excellent "read aloud" adventure, set in modern day London, mixing just enough Wizard of Oz and Mary Poppins fantasy to its sinister gothic nightmares. Underneath the streets lies an entirely separate and magical city, populated by all those unfortunate souls who have "fallen through the cracks in the world." From the rat-speakers to the sinister and ageless Mr. Croup and Mr. Vandemar, from real angels to horribly surreal tours of the historical significance of sewer masonry, Richard Mayhew finds more than enough risk and danger to worry about his own inner turmoil.