MLA
Marshall, Michael. Letter from Michael Marshall to His Sister, Margaret Marshall. Michael Was a Solider in the United States Army, 17th Infantry Regiment, Company A, and Died at the Battle of Globe Tavern in 1864 - 1860. 1860. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 23, Folder 20. https://digital.library.emory.edu/purl/774qjq2d5h-cor.
APA, 6th edition
Marshall, M. (1860). Letter from Michael Marshall to his sister, Margaret Marshall. Michael was a solider in the United States Army, 17th Infantry Regiment, Company A, and died at the Battle of Globe Tavern in 1864 - 1860 (Box 23, Folder 20) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/774qjq2d5h-cor.
Chicago
Marshall, Michael. “Letter from Michael Marshall to His Sister, Margaret Marshall. Michael Was a Solider in the United States Army, 17th Infantry Regiment, Company A, and Died at the Battle of Globe Tavern in 1864 - 1860.” Letters (correspondence). Civil War Collection, 1860. Box 23, Folder 20. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/774qjq2d5h-cor.