MLA
Marshall, Charles. Letter from Charles Marshall (Baltimore, Maryland) to General Marcus Wright, Regarding What Address to Use to Send a Book to Wright. Marshall Was a Colonel in the Confederate Army and Aide to General Robert E. Lee - December 7, 1880. 1880. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 9, Folder 12. https://digital.library.emory.edu/purl/525k98sg9g-cor.
APA, 6th edition
Marshall, C. (1880). Letter from Charles Marshall (Baltimore, Maryland) to General Marcus Wright, regarding what address to use to send a book to Wright. Marshall was a Colonel in the Confederate army and aide to General Robert E. Lee - December 7, 1880 (Box 9, Folder 12) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/525k98sg9g-cor.
Chicago
Marshall, Charles. “Letter from Charles Marshall (Baltimore, Maryland) to General Marcus Wright, Regarding What Address to Use to Send a Book to Wright. Marshall Was a Colonel in the Confederate Army and Aide to General Robert E. Lee - December 7, 1880.” Letters (correspondence). Civil War Collection, 1880. Box 9, Folder 12. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/525k98sg9g-cor.