MLA
Barrow, George W. Letter from George W. Barrow to His Brother from Prison at Camp Chase near Columbus, Ohio - April 11, 1865. 1865. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 22. https://digital.library.emory.edu/purl/617qnk99w4-cor.
APA, 6th edition
Barrow, G. W. (1865). Letter from George W. Barrow to his brother from prison at Camp Chase near Columbus, Ohio - April 11, 1865 (Box 1, Folder 22) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/617qnk99w4-cor.
Chicago
Barrow, George W. “Letter from George W. Barrow to His Brother from Prison at Camp Chase near Columbus, Ohio - April 11, 1865.” Letters (correspondence). Civil War Collection, 1865. Box 1, Folder 22. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/617qnk99w4-cor.