In "Ghost Map" Steven Johnson tells the story of the terrifying cholera epidemic that engulfed London in 1854, and the two unlikely heroes - anaesthetist Doctor John Snow and affable clergyman Reverend Henry Whitehead - who defeated the disease through a combination of local knowledge, scientific research and map-making. In telling their extraordinary story, Johnson also explores a whole world of ideas and connections, from urban terror to microbes, ecosystems to the Great Stink, cultural phenomena to street life. Re-creating a London full of dirt, dust heaps, slaughterhouses and scavengers, "Ghost Map" is about how huge populations live together, how cities can kill - and how they can save us.