MLA
Barrow, George W. Letter from George W. Barrow to "Dear Brother and Sisters" (Sydney, Emily, and Polly Ann); from Prison at Camp Douglas (Chicago, Illinois) - May 3, 1862. 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 22. https://digital.library.emory.edu/purl/948sbcc3gz-cor.
APA, 6th edition
Barrow, G. W. (1862). Letter from George W. Barrow to "Dear Brother and Sisters" (Sydney, Emily, and Polly Ann); from prison at Camp Douglas (Chicago, Illinois) - May 3, 1862 (Box 1, Folder 22) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/948sbcc3gz-cor.
Chicago
Barrow, George W. “Letter from George W. Barrow to ‘Dear Brother and Sisters’ (Sydney, Emily, and Polly Ann); from Prison at Camp Douglas (Chicago, Illinois) - May 3, 1862.” Letters (correspondence). Civil War Collection, 1862. Box 1, Folder 22. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/948sbcc3gz-cor.