Book excerpt: The only way to truly discover a city, they say, is on foot. Taking this to extremes, Mark Mason sets out to walk the entire length of the London Underground - overground - passing every station on the way. In a story packed with historical trivia, personal musings and eavesdropped conversations, Mark learns how to get the best gossip in the City, where to find a pint at 7am, and why the Bank of England won't let you join the M11 northbound at Junction 5. He has an East End cup of tea with the Krays' official biographer, discovers what cabbies mean by 'on the cotton', and meets the Archers star who was the voice of 'Mind the Gap'. Over the course of several hundred miles, Mark contemplates London's contradictions as well as its charms. He gains insights into our fascination with maps and sees how walking changes our view of the world. Above all, in this love letter to a complicated friend, he celebrates the sights, sounds and soul of the greatest city on earth.