MLA
Keen, Eleanor H. Letter by Eleanor H. Keen (Petersburg, Virginia) to Her Daughter - February 1, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 25. https://digital.library.emory.edu/purl/378tb2rcnw-cor.
APA, 6th edition
Keen, E. H. (1861). Letter by Eleanor H. Keen (Petersburg, Virginia) to her daughter - February 1, 1861 (Box 4, Folder 25) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/378tb2rcnw-cor.
Chicago
Keen, Eleanor H. “Letter by Eleanor H. Keen (Petersburg, Virginia) to Her Daughter - February 1, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 4, Folder 25. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/378tb2rcnw-cor.