MLA
Gonne, Maud, 1866-1953. Letter written by Maud Gonne to William Butler Yeats - February 15 [1906]. February 1906. 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/699n2z370m-cor.
APA, 6th edition
Gonne, Maud, 1866-1953. (February 1906)[Letter written by Maud Gonne to William Butler Yeats - February 15 [1906]]. 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/699n2z370m-cor.
Chicago
Gonne, Maud, 1866-1953, Letter written by Maud Gonne to William Butler Yeats - February 15 [1906], February 1906, 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/699n2z370m-cor.