The Government Documents collection is heavily used by students in urban and public affairs. The U.S. Census of Population and Housing is a key resource, as are several census publications which focus on cities and counties specifically:

County and City Data Book

Combines selected census data (economic, population and housing) and tabulates by country, region, state, county, city (25,000 or more), and place (2500 or more).
Fed Docs. C3.134/2: C83/2 (Most recent year available: 2000 in Fed Doc Ready Reference.) Earlier years shelved in Fed Doc Reference.
C 3.134/2-1: CD-ROM (Most recent year available: 1994) Ask at Reference Desk.

USA Counties

Provides detailed information on population, economics, education, health, elections, etc. Includes spans of years.
C 3.134/6: CD-ROM (Most recent year available: 1998.) Ask at Reference Desk.

County Business Patterns

Provides economic and employment information at the state and county level, by SIC code.
Fed Docs. C3.204/3 (Most recent year available: 1997.) Shelved on 3rd floor.
C 3.204/4: CD-ROM (Most recent year available: 2000/01.) Ask at Reference.