You can find some primary sources in books. Below is a list of relevant subject headings. When searching for these in the catalog, try adding "sources", "documents", "correspondence", "letters", or "writings" to your search.
DPLA: A portal to digitized material made available by America's public libraries, museums, and archives.
Internet Archive: A digital library of Internet sites and other cultural artifacts in digital form. Includes digitized books, periodicals, media, and more.
American Memory: Provides free and open access through the Internet to written and spoken words, sound recordings, still and moving images, prints, maps, and sheet music that document the American experience.
Avalon Project: Documents from pre-18th through the 20th century relevant to the fields of law, history, economics, politics, diplomacy, and government.
Making of America (via University of Michigan): Primary sources in American social history for the 19th century.
World Digital Library: Primary sources from all countries and cultures.