MLA
Mills, J. B. Letter from J. B. Mills (Camp near Nashville) to the Father of Joseph W. Ward Informing Him of His Son's Death at the Battle of Franklin. J. B. Mills Was in Company M of the 1st Mississippi Cavalry (Wood's Regiment) - December 8, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 7, Folder 12. https://digital.library.emory.edu/purl/825x69p9f9-cor.
APA, 6th edition
Mills, J. B. (1864). Letter from J. B. Mills (Camp near Nashville) to the father of Joseph W. Ward informing him of his son's death at the Battle of Franklin. J. B. Mills was in Company M of the 1st Mississippi Cavalry (Wood's Regiment) - December 8, 1864 (Box 7, Folder 12) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/825x69p9f9-cor.
Chicago
Mills, J. B. “Letter from J. B. Mills (Camp near Nashville) to the Father of Joseph W. Ward Informing Him of His Son's Death at the Battle of Franklin. J. B. Mills Was in Company M of the 1st Mississippi Cavalry (Wood's Regiment) - December 8, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 7, Folder 12. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/825x69p9f9-cor.