Presidential libraries have developed into an invaluable resource since President Franklin Roosevelt donated his personal and Presidential papers to the Federal Government in 1939, which started the Presidential Library system. The 14 presidential libraries, which store records for each President since Herbert Hoover, have been built with private funds but are maintained with public funds and administered by the Office of Presidential Libraries of the National Archives and Records Administration. The Presidential Records Act of 1978, as well as several Presidential Library Acts and executive orders, have established the rules governing access to the records. The sum total of all these rules and orders is that presidential records are owned by the public and the public is allowed access to these records under the Freedom of Information Act.
For researchers, the most immediately useful aspect of Presidential libraries are that they contain files created by and about employees of past administrations. For example, files from the George W. Bush presidential library were combed over by researchers engaged in the Kavanaugh Supreme Court nomination battle. Researchers should apply this same technique to any private collection that has been donated to a library: If a private party employed someone and then donated their collection to a library, whatever files they may have maintained on their prior employees becomes subject to whatever rules govern the collection.
Each of the Presidential Libraries maintains its own website – all linked here – which include guidance and instructions on searching the collections. Many of the older libraries have searchable databases on their websites, such as John F. Kennedy Presidential Library, the Nixon library and the Gerald R. Ford Presidential Library and Museum. The more recent libraries, however, are still in the process of being archived and typically require Freedom of Information Act (FOIA) requests to access large parts of their collections. The libraries offer different tips and resources for researchers. The Ronald Reagan Presidential Library & Museum, for instance, includes an exhaustive index of all the records maintained in the library. The George W. Bush Library includes an index of prior FOIA requests they’ve answered going back to 2014.
Keep in mind that presidential and private library collections evolve and grow as recently declassified or donated materials are indexed and made available so be on the lookout for material related to research subjects that might be newly public.