There's no one thing, but for historians, it's generally something that was created at the time of the event. Try searching using terms like: biography [also use for autobiographies], newspapers, "personal papers" [use for archival material], correspondence [use for letters], personal diaries, photographs, videos, interviews, pamphlets, artwork, sources [a common one!], and many other types of things!
A reference work can help you become acquainted with the literature on a topic if you are not aware of any works on your topic. The bibliographies of subject entries point you to the core work on a topic (as current as the publication of the book).
ADVANCED CATALYST SEARCH: Subject (contains) [general keyword] + bibliography
Questions? Contact reference@carleton.edu
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