Inside Google Books
The Inside Google Books blog has produced a number of interesting maps based on the frequency of locations mentioned in books during different decades in the 19th century.
The maps below show the number of times a location is mentioned across all of the books in Google Books Search. The maps were created using MapReduce and Bigtable
As the Inside Google Books blog points out the maps show the growth and westward expansion of the United States in the 19th century.