I read a more light-weight book this time, a nice one: Beneath the Metropolis, by Alex Marshall.
The book describes the underground features of modern cities, such as sewer systems, subways, tunnels etc. It is rich is nice details, such as the fact that the Waldorf Astoria Hotel in New York has a private metro station (kind of), and that there is a secret subway system under the regular metro in Moscow ("Metro 2").
It's a beautiful book, with a lot of maps, time lines and nice pictures.