MLA

Parker, Peter, 1804-1888. Courcy, Marquis de, to Peter Parker. n.d.. https://collections.library.yale.edu/catalog/16808616.

APA, 6th edition

Parker, Peter, 1804-1888. (n.d.)[Courcy, Marquis de, to Peter Parker]. https://collections.library.yale.edu/catalog/16808616.