The United States Government Manual - "official handbook of the Federal Government":

• Searchable current & updated edition via U.S. government
• Archived browseable editions (1995/96 to current) U.S. Gov't Manual Collection via the GPO
• Historical editions (1935 to current) via Hein Online (PDFs)
• In print (1977/78 to current) KF5101 .A32

Federal Regulatory Directory - agency organizational structures, background and history, incumbents for some positions, and contact information:

• In print (1979/80 to current) KF5407 .F43

Federal Government Agency Directory - Browseable or searchable list, via Louisana State University (free, open web resource).

General sources for agency related materials (subscription resources, individual password required):

• Lexis Advance - select Content Type = Administrative & Agency Materials > Federal > choose an Agency
• Westlaw Edge - select Content types = Administrative Decisions & Guidance > Federal
• Westlaw Edge - select Federal Materials > Federal Administrative Decisions & Guidance

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