MLA

Haas, Robert Bartlett (1916-2010). Twelve letters from Hal Levy of the Conference Press regarding the publication of What Are Masterpieces?. Box 5. 1936-1985, undated. https://collections.library.yale.edu/catalog/17173430.

APA, 6th edition

Haas, Robert Bartlett (1916-2010). (["Box 5"])(1936-1985, undated)[Twelve letters from Hal Levy of the Conference Press regarding the publication of What Are Masterpieces?]. https://collections.library.yale.edu/catalog/17173430.