MLA
Cooke, John Esten, 1830-1886. Letter from John Esten Cooke (Millwood, Virginia) to Philip Gates (New York) regarding Cooke's "Life of Robert E. Lee" - March 31, 1871. March 31, 1871. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/044bnzs87t-cor.
APA, 6th edition
Cooke, John Esten, 1830-1886. (March 31, 1871)[Letter from John Esten Cooke (Millwood, Virginia) to Philip Gates (New York) regarding Cooke's "Life of Robert E. Lee" - March 31, 1871]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/044bnzs87t-cor.
Chicago
Cooke, John Esten, 1830-1886, Letter from John Esten Cooke (Millwood, Virginia) to Philip Gates (New York) regarding Cooke's "Life of Robert E. Lee" - March 31, 1871, March 31, 1871, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/044bnzs87t-cor.