But the book's much more than an account of Snow's efforts and his map. Though those are at the center, Johnson uses them as a lens through which to examine everything from a history of cholera to sewer development to urbanization to the danger of nuclear terrorism. While all of the wide-ranging tangents aren't as interesting or as useful as others, for the most part the book makes for a good read.
The most interesting part to me, though, was the central account of Snow's original analysis of the epidemic (and his earlier work with other outbreaks), and the reception of his hypothesis by the local and scientific communities. Because of the importance of the map to the book, I was very surprised that Johnson doesn't include a good reproduction of it; seems like that would have been a no-brainer. Even without it, though, the argument about the importance of Snow's efforts is a good one.