MLA

Matthiessen, F. O. (Francis Otto), 1902-1950. Letters from Cheney to Matthiessen, letters 805-1549. Box 18. 1929 March 1-1941 December 23. https://collections.library.yale.edu/catalog/2040575.

APA, 6th edition

Matthiessen, F. O. (Francis Otto), 1902-1950. (["Box 18"])(1929 March 1-1941 December 23)[Letters from Cheney to Matthiessen, letters 805-1549]. https://collections.library.yale.edu/catalog/2040575.