MLA

Horn, Charles Frederick, 1762-1830. [Music book]. Box 818. [circa 1790]. https://collections.library.yale.edu/catalog/10000242.

APA, 6th edition

Horn, Charles Frederick, 1762-1830. (["Box 818"])([circa 1790])[[Music book]]. https://collections.library.yale.edu/catalog/10000242.