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Based on Sabin's Bibliotheca Americana: A Dictionary of Books Relating to America from Its Discovery to the Present Time. Full text books, pamphlets, broadsides & documents, from sermons & tracts to legislation & literature. Covers all the Americas.
The single best way to identify relevant primary source material is to read widely in the secondary literature on your topic; scrutinize the notes and bibliography for published primary sources that you can access.
As you search in the online catalog, watch for the words "personal narratives," "diaries," "interviews," "correspondence," "pamphlets," and "sources" as part of the subject headings; these may point you to published primary sources in the library collections.
Other clues may be buried in the title, authorship, date of publication, etc.