MLA
Yeats, W. B. (William Butler), 1865-1939. Letter written by William Butler Yeats to Maud Gonne - April 13, 1910. April 13, 1910. 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/969866t3xs-cor.
APA, 6th edition
Yeats, W. B. (William Butler), 1865-1939. (April 13, 1910)[Letter written by William Butler Yeats to Maud Gonne - April 13, 1910]. 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/969866t3xs-cor.
Chicago
Yeats, W. B. (William Butler), 1865-1939, Letter written by William Butler Yeats to Maud Gonne - April 13, 1910, April 13, 1910, 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/969866t3xs-cor.