To find out which cities are located in a county, try NACO's City-County Search. You can also look in the Rand McNally Commercial Atlas & Marketing Guide or another good road atlas.
The Atlas of Historical County Boundaries not only shows you the current boundaries of each county in the U.S., it explains how each county's boundaries changed over time.
You can look up the county for a particular address in the U.S. using the Census Bureau's American Factfinder. MelissaData posts a database of zip codes found in each county.
To find out the population, county seat/courthouse or land area, of a particular country, look in The World Almanac or use NACO's City-County Search. Population is also available in the "Counties/Area and Population" section of the Population page of the online National Data Book, in the Census Bureau's County and City Data Book and in the green pages of the Judicial Staff Directory.
For statistical information, see the Census Bureau's County and City Data Book, the County and City Extra and/or the States and Local Areas section of the online National Data Book.
You can check the financial status of a county that issues bonds by pulling their annual reports from the MSRB's EMMA database. You may also find the county's the debt ratings. Alternatively, you can get debt ratings from the individual rating agency web sites listed in the "Debt Ratings" entry.
For county laws and codes, see "Local Laws."