MLA
Gonne, Maud, 1866-1953. Letter written by Maud Gonne to William Butler Yeats - [September 1927]. September 1927. 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/822h70s183-cor.
APA, 6th edition
Gonne, Maud, 1866-1953. (September 1927)[Letter written by Maud Gonne to William Butler Yeats - [September 1927]]. 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/822h70s183-cor.
Chicago
Gonne, Maud, 1866-1953, Letter written by Maud Gonne to William Butler Yeats - [September 1927], September 1927, 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/822h70s183-cor.