MLA
Walker, William Henry Talbot, 1816-1864. Letter from William Henry Talbot Walker (West Point, N. Y.) to Robert H. Pruyn, regarding artillery drills at the United States Military Academy at West Point [Although the Letter is addressed to Robert J. Prun, the address is actually Robert Hewson Pruyn who was Adjutant General of New York militia]. June 21, 1857. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/2085hqc0pb-cor.
APA, 6th edition
Walker, William Henry Talbot, 1816-1864. (June 21, 1857)[Letter from William Henry Talbot Walker (West Point, N. Y.) to Robert H. Pruyn, regarding artillery drills at the United States Military Academy at West Point [Although the Letter is addressed to Robert J. Prun, the address is actually Robert Hewson Pruyn who was Adjutant General of New York militia]]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/2085hqc0pb-cor.
Chicago
Walker, William Henry Talbot, 1816-1864, Letter from William Henry Talbot Walker (West Point, N. Y.) to Robert H. Pruyn, regarding artillery drills at the United States Military Academy at West Point [Although the Letter is addressed to Robert J. Prun, the address is actually Robert Hewson Pruyn who was Adjutant General of New York militia], June 21, 1857, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/2085hqc0pb-cor.