MLA
Scott, Alexander. Letter from Alexander Scott to His Wife, Sarah Ann Scott. Alexander Scott Was a Prisoner of War at Camp Chase and Camp Dennison in Ohio. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 15. https://digital.library.emory.edu/purl/767wm37qk6-cor.
APA, 6th edition
Scott, A. (1863). Letter from Alexander Scott to his wife, Sarah Ann Scott. Alexander Scott was a prisoner of war at Camp Chase and Camp Dennison in Ohio (Box 6, Folder 15) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/767wm37qk6-cor.
Chicago
Scott, Alexander. “Letter from Alexander Scott to His Wife, Sarah Ann Scott. Alexander Scott Was a Prisoner of War at Camp Chase and Camp Dennison in Ohio.” Letters (correspondence). Civil War Collection, 1863. Box 6, Folder 15. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/767wm37qk6-cor.