MLA

Wharton, Edith, 1862-1937. Howells, William Dean. Box 26, folder 802. 1900-19. https://collections.library.yale.edu/catalog/17111152.

APA, 6th edition

Wharton, Edith, 1862-1937. (["Box 26, folder 802"])(1900-19)[Howells, William Dean]. https://collections.library.yale.edu/catalog/17111152.