MLA
Maxwell, G. Troup. Letter from G. Maxwell (New Castle, Delaware) to Major William F. Slaton, Mentioning an Incident Occurring at the Battle of Lookout Mountain and His Experiences at the Prison Camp on Johnson's Island - May 12, 1874 (Typescript). 1874. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 9, Folder 14. https://digital.library.emory.edu/purl/4988gthv7f-cor.
APA, 6th edition
Maxwell, G. T. (1874). Letter from G. Maxwell (New Castle, Delaware) to Major William F. Slaton, mentioning an incident occurring at the Battle of Lookout Mountain and his experiences at the prison camp on Johnson's Island - May 12, 1874 (typescript) (Box 9, Folder 14) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4988gthv7f-cor.
Chicago
Maxwell, G. Troup. “Letter from G. Maxwell (New Castle, Delaware) to Major William F. Slaton, Mentioning an Incident Occurring at the Battle of Lookout Mountain and His Experiences at the Prison Camp on Johnson's Island - May 12, 1874 (Typescript).” Letters (correspondence). Civil War Collection, 1874. Box 9, Folder 14. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4988gthv7f-cor.