United States Government Manual

The United States Government Manual is the official handbook of the Federal Government.  It's maintained by the Federal Register Office of the National Archives and Records Administration:

http://www.usgovernmentmanual.gov/Home.aspx

The Manual provides the text of the Declaration of Independence and Constitution, and a description of Federal agencies, boards, commissions, and so on, in all three branches of the government.  It could be considered the "owner's manual" for the U.S. Government.  In fact, if the U.S. was a car, this manual would probably be kept in the glove compartment.