MLA

Phelps, William Lyon, 1865-1943. Cather, Willa. Box 11. 1943 May 29. https://collections.library.yale.edu/catalog/33220153.

APA, 6th edition

Phelps, William Lyon, 1865-1943. (["Box 11"])(1943 May 29)[Cather, Willa]. https://collections.library.yale.edu/catalog/33220153.