MLA
Howard, Monroe. Letter from Monroe Howard to His Brother, Dr. Thomas Henry Howard - January 20, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 3, Folder 64. https://digital.library.emory.edu/purl/047rn8pm0q-cor.
APA, 6th edition
Howard, M. (1863). Letter from Monroe Howard to his brother, Dr. Thomas Henry Howard - January 20, 1863 (Box 3, Folder 64) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/047rn8pm0q-cor.
Chicago
Howard, Monroe. “Letter from Monroe Howard to His Brother, Dr. Thomas Henry Howard - January 20, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 3, Folder 64. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/047rn8pm0q-cor.