MLA
James [unknown last name]. Letter from James [unknown last name] (Camp Leon, Florida) to his sister, "Dear Sallie" - August 25, 1864. August 25, 1864. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/061djh9x1x-cor.
APA, 6th edition
James [unknown last name]. (August 25, 1864)[Letter from James [unknown last name] (Camp Leon, Florida) to his sister, "Dear Sallie" - August 25, 1864]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/061djh9x1x-cor.
Chicago
James [unknown last name], Letter from James [unknown last name] (Camp Leon, Florida) to his sister, "Dear Sallie" - August 25, 1864, August 25, 1864, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/061djh9x1x-cor.