Started as a, big surprise, library for Congress to use. Their original collection got burned so Thomas Jefferson agreed to sell his own library to them. His collection is on display and is quite impressive, still.
As a geek, I absolutely loved the LoC. I even got a reader's card so that I could go into the reading room. Only serious researchers are aloud to get a card. I am serious.
Anyhow words can't express how impressive the building is, inside and out. It's a temple of the intellectual achievement of man.
Does the Library have a copy of every book published in the United States?
No, but it does have more than 33 million books and printed materials, as well as more than 111 million maps, manuscripts, photographs, films, audio and video recordings, prints and drawings, and other special collections.
