Primary means written at or near the time of the historical event.
Secondary means written many years later, probably by a scholar researching the historical event.
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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" to your search.
Books are organized in the library by the Library of Congress Classification System. LC Subject Headings are assigned to every book. They will help you locate more relevant books on your topic.