MLA
Taylor, B. T. Letter from B. T. Taylor (Camp Troup Artillery) to "Dear Sister" - October 20, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 43. https://digital.library.emory.edu/purl/617qnk99k0-cor.
APA, 6th edition
Taylor, B. T. (1864). Letter from B. T. Taylor (Camp Troup Artillery) to "Dear Sister" - October 20, 1864 (Box 6, Folder 43) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/617qnk99k0-cor.
Chicago
Taylor, B. T. “Letter from B. T. Taylor (Camp Troup Artillery) to ‘Dear Sister’ - October 20, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 6, Folder 43. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/617qnk99k0-cor.