MLA
Marcum, E. W. Letter from E. W. Marcum (Camp near Cartersville, Georgia) to "Dear Cousin" - January 24, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 58. https://digital.library.emory.edu/purl/825x69p9dm-cor.
APA, 6th edition
Marcum, E. W. (1864). Letter from E. W. Marcum (camp near Cartersville, Georgia) to "Dear cousin" - January 24, 1864 (Box 4, Folder 58) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/825x69p9dm-cor.
Chicago
Marcum, E. W. “Letter from E. W. Marcum (Camp near Cartersville, Georgia) to ‘Dear Cousin’ - January 24, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 4, Folder 58. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/825x69p9dm-cor.