The Library of Congress is the largest library in the world, with over 150 million books and research items, including one of only three perfect Gutenberg Bibles on display. The Beaux-Arts building has many stunning architectural details, and is highly ornamented with murals, sculptures, marble floors, and vaulted ceilings. It's recommended to take one of the one-hour-long guided tours, as the building and its contents can be overwhelming.  Remember that anyone with a US Driver's License or passport can become a member here, and make use of the research facilities, though only librarians and members of Congress can go into the stacks.