The Library of Congress is the largest library in the world, housed in three buildings on Capitol Hill, of which the Thomas Jefferson Building (completed 1897) is the most spectacular. The building's great hall, with its elaborate mosaic floors, arched ceilings, and allegorical paintings, is one of the finest interior spaces in Washington and is open to the public without charge.
The Library's exhibitions in the ground floor galleries rotate regularly and draw on the extraordinary breadth of the collection — early maps, manuscript letters of founding fathers, first-edition literary texts, and much else. Guided tours are available. The reading rooms are technically for registered researchers, but the visitor gallery on the first floor allows you to look down on the Main Reading Room from above, which is experience enough.