Here is a very long list of references compiled by the Library of Congress sorted by category.
Online reference desk of the Library of Congress
What is the Library of Congress?
From their website:
The Library of Congress is the nation's oldest federal cultural institution and serves as the research arm of Congress. It is also the largest library in the world, with millions of Books, Recordings, Photos, Maps and manuscripts in its collections.
The Library's mission is to make its resources available and useful to the Congress and the American people and to sustain and preserve a universal collection of knowledge and creativity for future generations. The Office of the Librarian is tasked to set policy and to direct and support programs and activities to accomplish the Library's mission.