Encyclopedias and almanacs can be useful for statistics and brief descriptions of cities.
Britannica Online - Encyclopaedia Britannica Excellent source of statistics, maps and histories.
Wikipedia List of Cities entries with varying level of quality but usually a good source for references.
American Factfinder United States Census. Also see definition of Urban Areas here and United Nations - Urban and Rural and related sources in each.
U.S. Bureau of Labor Statistics good source of economic and employment data for cities and regions. Most countries have a similar source. See also the U.N. International Labour Organization
Infoplease Almanac Online almanac as well as an encyclopedia, dictionary, atlas
