Beckett, Samuel. Letter to Barbara Bray, 24 Mar. 1960. The Letters of Samuel Beckett, Vol. 3: 1957-1965, edited by George Craig et al., Cambridge UP, 2014, pp. 317-18.
Explain
Begin with the writer of the letter, the words “Letter to” and the recipient.
Add the date of the letter, if there is one.
Add the title of the collection, the editor (if any), and publication information.