MLA
Simpson, William D. Letter from William D. Simpson (Richmond, Virginia), to His Wife - February 17, 1865. 1865. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 23. https://digital.library.emory.edu/purl/433dz08mdz-cor.
APA, 6th edition
Simpson, W. D. (1865). Letter from William D. Simpson (Richmond, Virginia), to his wife - February 17, 1865 (Box 6, Folder 23) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/433dz08mdz-cor.
Chicago
Simpson, William D. “Letter from William D. Simpson (Richmond, Virginia), to His Wife - February 17, 1865.” Letters (correspondence). Civil War Collection, 1865. Box 6, Folder 23. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/433dz08mdz-cor.