MLA
Letter from the Secretary of War ... March 31, 1862 : [Communicating Copies of the Official Reports of the Battle of Bethel, on the 10th of June, 1861]. [publisher not identified], 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/030prr51sv-cor.
APA, 6th edition
Letter from the secretary of war ... March 31, 1862 : [communicating copies of the official reports of the battle of Bethel, on the 10th of June, 1861]. (1862). [publisher not identified]; Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/030prr51sv-cor.
Chicago
“Letter from the Secretary of War ... March 31, 1862 : [Communicating Copies of the Official Reports of the Battle of Bethel, on the 10th of June, 1861].” Civil War Imprints. Richmond: [publisher not identified], 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/030prr51sv-cor.