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 - January 26, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 15. https://digital.library.emory.edu/purl/4407wm38c4-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 - January 26, 1863 (Box 6, Folder 15) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4407wm38c4-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 - January 26, 1863.” 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/4407wm38c4-cor.