I’m reading London Under: the Secret History Under the Streets by Peter Ackroyd. It’s a nifty read, not too overwhelming with detail but rich in buried history. To complete the experience, Cities of the Underworld (Season one) explores the underside of London, among other cities. There is likely more underground reading if you start digging! –Lisa D.
Lisa is right—there are all sorts of subterranean tales to be found in the library catalog. A subject search for “Underground Areas” brings up the following:
Beneath the Metropolis: the Secret Lives of Cities by Alex Marshall
Beneath the Neon: life and death in the tunnels of Las Vegas by Matthew O’Brien
New York Underground: the anatomy of a city by Julia Solis
And for those seeking a fictional take on the worlds beneath our feet:
Neil Gaiman’s Neverwhere takes place in London Below, a subterranean city made up of long-forgotten parts of the 
historic London underground and populated by people who “fell through the cracks,” who live in an almost feudal society that includes outlaw and supernatural elements. Gaiman is now considered one of the leading lights in the world of fantastical fiction. Neverwhere was one of his first full-length novels and is still very difficult to surpass in the tightness and rightness of the meeting of fantasy and gritty realism.
For a local connection, The Shanghai Tunnel is a historical fiction/ mystery by Sharan Newman. It’s set in Portland during the 1860’s. The undergrounds in this book are both literal and of the criminal variety.
In the juvenile fiction section, The City of Ember by Jean DuPrau describes a civilization that for generations has lived 
in a city deep underground. The people have long since forgotten how to do anything but maintain the equipment and supplies that were left to them by those that fled underground all those years ago- they have even forgotten how to make fire. But now the food supplies are running low and the lights are going out and Doon and Lina, two teenagers, seem to be the only ones willing to risk finding a way to the surface.
