The text and labels of the online exhibitions of the Library can be useful secondary sources, but can also be a helpful place to find primary sources (especially documents and videos) by theme.
Guides to the Library's general collections of books and periodicals, as well as general subject guides prepared by Library of Congress reference librarians.
The Library's E-resources Online Catalog provides descriptions and access to a large number of freely-available secondary sources useful in researching a wide variety of topics.