MLA
Yeats, W. B. (William Butler), 1865-1939. Letter written by William Butler Yeats to Maud Gonne - January 14, 1907. January 14, 1907. 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/590dv41r5d-cor.
APA, 6th edition
Yeats, W. B. (William Butler), 1865-1939. (January 14, 1907)[Letter written by William Butler Yeats to Maud Gonne - January 14, 1907]. 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/590dv41r5d-cor.
Chicago
Yeats, W. B. (William Butler), 1865-1939, Letter written by William Butler Yeats to Maud Gonne - January 14, 1907, January 14, 1907, 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/590dv41r5d-cor.