In 1993, journalist Jennifer Toth published The Mole People, a book-length investigation of New York City's subway and rail tunnels and the mysterious civilization of homeless people dwelling there. Although I sensed much of the book was fictional, exaggerated, or based on statements that were never verified, I still found The Mole People very effective as a piece of writing, and had nightmares for weeks afterward based on the characters and places.

Subway obsessive Joseph Brennan (who does the GREAT "Abandoned Stations" site) has confirmed my suspicions -- he claims "every fact in this book that I can verify independently is wrong," and backs this up with some rather damning evidence. (Thanks to Chris Barrus for the link.)

On a related topic: If you've got a strong stomach for this sort of thing, check out these photos taken by transit workers just days after the WTC collapse. This one probably hit me the hardest.