MLA
Howard, Monroe. Letter from Monroe Howard (Jacksonville, Virginia) to His Brother, Dr. Thomas Henry Howard - November 4, 1860. 1860. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 3, Folder 64. https://digital.library.emory.edu/purl/68605qfvs5-cor.
APA, 6th edition
Howard, M. (1860). Letter from Monroe Howard (Jacksonville, Virginia) to his brother, Dr. Thomas Henry Howard - November 4, 1860 (Box 3, Folder 64) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/68605qfvs5-cor.
Chicago
Howard, Monroe. “Letter from Monroe Howard (Jacksonville, Virginia) to His Brother, Dr. Thomas Henry Howard - November 4, 1860.” Letters (correspondence). Civil War Collection, 1860. Box 3, Folder 64. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/68605qfvs5-cor.