If you want to know how to contact your senator or look up the address of a federal agency, these directories are a good place to start.
House Members Directory (house.gov)
Directories of Members of Congress
The Congressional Directory includes not only contact information for Congress, but also has several useful tables. The Congressional Pictorial Directory has photographs of members of Congress
The United States Government Manual is the official handbook of the federal government. In addition to listing addresses and phone numbers for offices in the government, the Manual provides comprehensive information on the agencies of the legislative, judicial, and executive branches.
United States Government Manual (govinfo-GPO)