MLA

Ernest Augustus, King of Hanover, 1771-1851. Music book. Box 824. undated. https://collections.library.yale.edu/catalog/10050487.

APA, 6th edition

Ernest Augustus, King of Hanover, 1771-1851. (["Box 824"])(undated)[Music book]. https://collections.library.yale.edu/catalog/10050487.