The Subterranean Railway - Christian Wolmar

I've picked up various titbits about the history of the London Underground over the years, but this skilfully put together overview has given me a far better understanding of the story. The random, uncoordinated nature of the early system explains many of the anomalies that exist to this day, and I liked the odd aside such as the Sudanese diplomat who described the atmosphere in the tunnels as "like a crocodile's breath".