In the United States, Congress and its associated agencies comprise the legislative branch. There are many types of congressional documentation, including bills, laws, hearings, and reports.
USA.gov has basic information on how a bill becomes a law. The Law Library of Congress has more detailed information on the legislative process.
Aside from books and articles, reports from the Congressional Research Service, the Congressional Budget Office, and the Government Accountability Office are excellent places to start. For example, the CRS report "The President’s Office of Science and Technology Policy (OSTP): Issues for Congress" cites legislation, executive branch documents, tables, and figures.