MLA

Sergeant, Elizabeth Shepley, 1881-1965. Cather, Willa. Box 1, folder 26. n.d.. https://collections.library.yale.edu/catalog/33220181.

APA, 6th edition

Sergeant, Elizabeth Shepley, 1881-1965. (["Box 1, folder 26"])(n.d.)[Cather, Willa]. https://collections.library.yale.edu/catalog/33220181.