MLA
Howard, Peter L. Letter from Peter L. Howard to His Brother, Dr. Thomas Henry Howard. Peter L. Howard May Have Served in the 5th Regiment, Georgia Cavalry - April 9, 1862. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 3, Folder 64. https://digital.library.emory.edu/purl/791sf7m1d1-cor.
APA, 6th edition
Howard, P. L. (1862). Letter from Peter L. Howard to his brother, Dr. Thomas Henry Howard. Peter L. Howard may have served in the 5th Regiment, Georgia Cavalry - April 9, 1862 (Box 3, Folder 64) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/791sf7m1d1-cor.
Chicago
Howard, Peter L. “Letter from Peter L. Howard to His Brother, Dr. Thomas Henry Howard. Peter L. Howard May Have Served in the 5th Regiment, Georgia Cavalry - April 9, 1862.” Letters (correspondence). Civil War Collection, 1862. Box 3, Folder 64. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/791sf7m1d1-cor.