MLA
Riboud, Alexis. Letter from Alexis Riboud to His Mother, Barbara Chase-Riboud (New York City, New York) - December 9, [No Year Provided]. 1975. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 10. https://digital.library.emory.edu/purl/7547pvmdr8-cor.
APA, 6th edition
Riboud, A. (1975). Letter from Alexis Riboud to his mother, Barbara Chase-Riboud (New York City, New York) - December 9, [No Year Provided] (Box 1, Folder 10) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/7547pvmdr8-cor.
Chicago
Riboud, Alexis. “Letter from Alexis Riboud to His Mother, Barbara Chase-Riboud (New York City, New York) - December 9, [No Year Provided].” Letters (correspondence). Barbara Chase-Riboud Papers, 1975. Box 1, Folder 10. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/7547pvmdr8-cor.