MLA

Matthiessen, F. O. (Francis Otto), 1902-1950. Letters from Russell Cheney to F. O. Matthiessen, letters 1-804. Box 17. 1924 September 1-1929 February 28. https://collections.library.yale.edu/catalog/2017619.

APA, 6th edition

Matthiessen, F. O. (Francis Otto), 1902-1950. (["Box 17"])(1924 September 1-1929 February 28)[Letters from Russell Cheney to F. O. Matthiessen, letters 1-804]. https://collections.library.yale.edu/catalog/2017619.