A Google engineer decided to make a map based on how many times each city is mentioned in books indexed by Google's Book Search. The result is the image above, with the bright green areas getting the most mentions. Presumably, Google searches more English-language texts, given the high distribution of green dots in English-speaking countries like the U.S. and India, compared to the vast white spaces in China, Russia, and Latin America.
Now, if they could only correlate the brightness of each dot to actual sales of the books in which they are mentioned. (Or better yet, make an interactive map that calls up the books in question whenever you roll your cursor over a given city).