MLA
Miles, James Jefferson. Letter from James Jefferson Miles to an Unknown Recipient. Miles Served in the 3rd Mississippi Infantry Battalion, Later the 45th Mississippi Infantry Regiment - April 1, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 5, Folder 17. https://digital.library.emory.edu/purl/0278sf7n27-cor.
APA, 6th edition
Miles, J. J. (1864). Letter from James Jefferson Miles to an unknown recipient. Miles served in the 3rd Mississippi Infantry Battalion, later the 45th Mississippi Infantry Regiment - April 1, 1864 (Box 5, Folder 17) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/0278sf7n27-cor.
Chicago
Miles, James Jefferson. “Letter from James Jefferson Miles to an Unknown Recipient. Miles Served in the 3rd Mississippi Infantry Battalion, Later the 45th Mississippi Infantry Regiment - April 1, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 5, Folder 17. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/0278sf7n27-cor.