MLA
Barnett, D. T. Letter from D. T. Barnett (near Dalton, Georgia) to "Dear Mother, Brother, and Sisters" - April 26, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 20. https://digital.library.emory.edu/purl/665h9w0wsj-cor.
APA, 6th edition
Barnett, D. T. (1864). Letter from D. T. Barnett (near Dalton, Georgia) to "Dear Mother, Brother, and Sisters" - April 26, 1864 (Box 1, Folder 20) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/665h9w0wsj-cor.
Chicago
Barnett, D. T. “Letter from D. T. Barnett (near Dalton, Georgia) to ‘Dear Mother, Brother, and Sisters’ - April 26, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 1, Folder 20. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/665h9w0wsj-cor.