The Library of Congress is the research library of the United States Congress, de facto national library of the United States, and the oldest federal cultural institution in the United States. Located in three buildings in Washington, D.C., it is the largest library in the world by shelf space and number of books. The head of the Library is the Librarian of Congress, currently James H. Billington.
The Library of Congress was established by Congress in 1800, and was housed in the United States Capitol for most of the 19th century. more information here