MLA

Audubon, John Woodhouse, 1812-1862, Lambert, John B. (John Booth), -1849, Lambert, John S, Trask, John B. (John Boardman), 1824-1879. John S. Lambert and John B. Lambert letters relating to the California gold rush. [19--]. https://collections.library.yale.edu/catalog/17259075.

APA, 6th edition

Audubon, John Woodhouse, 1812-1862, & Lambert, John B. (John Booth), -1849, & Lambert, John S, & Trask, John B. (John Boardman), 1824-1879. ([19--])[John S. Lambert and John B. Lambert letters relating to the California gold rush]. https://collections.library.yale.edu/catalog/17259075.