Please note: these sources are for use with the library exercise only. For your normal research, use these more comprehensive guides:
For gathering background information, it is usually best to start with secondary sources. Take thorough notes when using them: dates, names, Congress and session numbers, bill numbers, titles and numbers of Public Laws, citations to court cases, etc. Those notes will make your research vastly more efficient, especially when you turn to primary sources.
United States Government Manual via HeinOnline, 1935-2014
United States Government Manual (print and online)
United States Government Manual (current)
Govt Ref Desk AE 2/108.2:date
Govt Index AE 2/108.2:date
Presidential libraries, from the National Archives and Records Administration (NARA)