MLA
Yeats, W. B. (William Butler), 1865-1939. Letter written by William Butler Yeats to Maud Gonne - August 18 [1918]. August 18, 1918. 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/948sbcc4rs-cor.
APA, 6th edition
Yeats, W. B. (William Butler), 1865-1939. (August 18, 1918)[Letter written by William Butler Yeats to Maud Gonne - August 18 [1918]]. 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/948sbcc4rs-cor.
Chicago
Yeats, W. B. (William Butler), 1865-1939, Letter written by William Butler Yeats to Maud Gonne - August 18 [1918], August 18, 1918, 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/948sbcc4rs-cor.