MLA
William [unknown last name]. Letter from William [unknown last name] (Camp near Dalton, Georgia), to his sister - January 25, 1864. January 25, 1864. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4816m906rd-cor.
APA, 6th edition
William [unknown last name]. (January 25, 1864)[Letter from William [unknown last name] (Camp near Dalton, Georgia), to his sister - January 25, 1864]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4816m906rd-cor.
Chicago
William [unknown last name], Letter from William [unknown last name] (Camp near Dalton, Georgia), to his sister - January 25, 1864, January 25, 1864, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4816m906rd-cor.