MLA
Gonne, Maud, 1866-1953. Letter written by Maud Gonne to William Butler Yeats - April 3 [1921]. April 3, 1921. 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/211kh18cf0-cor.
APA, 6th edition
Gonne, Maud, 1866-1953. (April 3, 1921)[Letter written by Maud Gonne to William Butler Yeats - April 3 [1921]]. 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/211kh18cf0-cor.
Chicago
Gonne, Maud, 1866-1953, Letter written by Maud Gonne to William Butler Yeats - April 3 [1921], April 3, 1921, 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/211kh18cf0-cor.