Start with a keyword search in the catalog. Once you have located a relevant item, you can look at the Library of Congress Subject Headings within the full record tab and follow those links to find related books.
To find primary resources, you can add any of the following keywords to your search: letters, diaries, speeches, newspaper articles, autobiographies, oral histories, government and organizational records, statistical data, maps, photographs, motion pictures, sound recordings, advertisements, and artifacts.
Here are some sample subject headings from the library catalog. You can also add your country as a keyword to refine your search.
For keyword searches, try:
Use the "search within results" checkbox to add keywords and increase specificity.