MLA List of Works Cited: Other Sources - Published Letter
Cite a published letter the same way as a work in an anthology. After the name of the author, include any title the editor gives the letter and the date. Add the page numbers for the letter at the end of the citation.
Bishop, Elizabeth. "To Robert Lowell." 26 Nov. 1951. One Art: Letters. Ed. Robert Giroux. New York:
Farrar, 1994. 224-26.