A state library is usually a library established by the government of a state to serve as the preeminent repository of information for that state.
Chief Officers of State Library Agencies (COSLA) is an independent organization of the chief officers of state and territorial agencies designated as the state library administrative agency and responsible for statewide library development. For quick access to directory information on each state library agency, use the interactive map.
Institute of Museum and Library Services (IMLS) -- List of links to the websites of all fifty state libraries in the United States as well as the head library agencies of the U.S. territories, compiled by IMLS
The Directory of State Libraries provides a list of State Libraries that is compiled by a private group.
Himmel, Ethel E., William James Wilson, and GraceAnne A. DeCandido. The Functions and Roles of State Library Agencies. Chicago, Ill: American Library Association, 2000.