Available dates: 1851-2015
"This collection provides the public with access to research products produced by the Congressional Research Service (CRS) for the United States Congress. By law, CRS works exclusively for Congress, providing timely, objective, and authoritative research and analysis to committees and Members of both the House and Senate, regardless of political party affiliation. As a legislative branch agency within the Library of Congress, CRS has been a valued and respected resource on Capitol Hill for more than a century." For decades CRS did not make their reports available to the public. They were distributed only to members of Congress, but Congress could distribute the documents however they wished, which is why reports can be found on various websites. Several years ago, CRS Reports became accessible to all from this web site.
To get a sense of the scope of the reports, try a simple search on "immigration." Take a look at the report "Primer on U.S. Immigration Policy."