MLA
Boyd, W. S. Letter from W. S. Boyd (Campbell) to Confederate General Wilmot Gibbes De Saussure Saying That He Has No Corn or Fodder to Send Him and That They Would Be Difficult to Find - February 3, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 3, Folder 3. https://digital.library.emory.edu/purl/088qbzkj13-cor.
APA, 6th edition
Boyd, W. S. (1864). Letter from W. S. Boyd (Campbell) to Confederate General Wilmot Gibbes De Saussure saying that he has no corn or fodder to send him and that they would be difficult to find - February 3, 1864 (Box 3, Folder 3) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/088qbzkj13-cor.
Chicago
Boyd, W. S. “Letter from W. S. Boyd (Campbell) to Confederate General Wilmot Gibbes De Saussure Saying That He Has No Corn or Fodder to Send Him and That They Would Be Difficult to Find - February 3, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 3, Folder 3. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/088qbzkj13-cor.