MLA
Clark, Walter H. Letter from Walter H. Clark to William H. Clark - April 5, 1858. 1858. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 21. https://digital.library.emory.edu/purl/68605qfvrc-cor.
APA, 6th edition
Clark, W. H. (1858). Letter from Walter H. Clark to William H. Clark - April 5, 1858 (Box 2, Folder 21) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/68605qfvrc-cor.
Chicago
Clark, Walter H. “Letter from Walter H. Clark to William H. Clark - April 5, 1858.” Letters (correspondence). Civil War Collection, 1858. Box 2, Folder 21. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/68605qfvrc-cor.