(code courtesy of MADGIC, Carleton University)
The govinfo site provides public access to official publications from the executive, legislative and judicial branches of the U.S. federal government. The scope is broad, but coverage is generally better the more recent the timeframe. You can get a better idea of what is available by browsing the collection by category. The govinfo site replaced FDSys in December 2018.
The The United States Government Manual contains descriptions for the services and programs of the various agencies of the three branches of government, including lists of individuals holding key leadership roles. It also contains a history of U.S. government agency organizational changes.