When the Library of Congress started, it was not big. Books were not common. Many people could not read. The library started with law books. It was for members of Congress to research things.
Thomas Jefferson said Congress needed to learn more than just laws. They needed to learn about history, people from other lands, and the world. When the library was burned during the War of 1812, most of the books were lost. Jefferson offered to sell his own much larger library to America.