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The Library of Congress  (http://www.loc.gov/index.html) in Washington D.C. has over 33 million books and 12 million photographs, millions of maps, sheet music, and other manuscripts.

Its holdings make it largest library in the western world; and having been founded in 1800, it is the oldest federal cultural institution in the country.

Source of information:
The World Almanac 2010, original source
The Refdesk 2 Sept 2010, indirect reference source
The Library of Congress (http://www.loc.gov/index.html)