MLA
Gonne, Maud, 1866-1953. Letter written by Maud Gonne to William Butler Yeats - August 15 [1896]. August 15, 1896. Maud Gonne and W.B. Yeats Papers, circa 1890-1938. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/679573n851-cor.
APA, 6th edition
Gonne, Maud, 1866-1953. (August 15, 1896)[Letter written by Maud Gonne to William Butler Yeats - August 15 [1896]]. Maud Gonne and W.B. Yeats Papers, circa 1890-1938, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/679573n851-cor.
Chicago
Gonne, Maud, 1866-1953, Letter written by Maud Gonne to William Butler Yeats - August 15 [1896], August 15, 1896, Maud Gonne and W.B. Yeats Papers, circa 1890-1938, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/679573n851-cor.