MLA
Willis, Samuel C. Letter from Samuel C. Willis (Front Royal, Virginia) to A. L. Cook. Willis Had Been a Member of the 1st Rhode Island Cavalry and Had Witnessed the Killing of Some of Mosby's Rangers at Front Royal on September 23, 1864 - 1864(Photocopy). 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 20, Folder 39. https://digital.library.emory.edu/purl/047rn8pm4r-cor.
APA, 6th edition
Willis, S. C. (1864). Letter from Samuel C. Willis (Front Royal, Virginia) to A. L. Cook. Willis had been a member of the 1st Rhode Island Cavalry and had witnessed the killing of some of Mosby's Rangers at Front Royal on September 23, 1864 - 1864(photocopy) (Box 20, Folder 39) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/047rn8pm4r-cor.
Chicago
Willis, Samuel C. “Letter from Samuel C. Willis (Front Royal, Virginia) to A. L. Cook. Willis Had Been a Member of the 1st Rhode Island Cavalry and Had Witnessed the Killing of Some of Mosby's Rangers at Front Royal on September 23, 1864 - 1864(Photocopy).” Letters (correspondence). Civil War Collection, 1864. Box 20, Folder 39. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/047rn8pm4r-cor.