MLA
Early, John, 1786-1873. Letter from John Early (Richmond, Virginia) to Robert Paine (Aberdeen, Mississippi) - August 27, 1862. August 27, 1862. Robert Paine Papers, 1828-1882. Pitts Theology Library. https://digital.library.emory.edu/purl/24298sf8rj-cor.
APA, 6th edition
Early, John, 1786-1873. (August 27, 1862)[Letter from John Early (Richmond, Virginia) to Robert Paine (Aberdeen, Mississippi) - August 27, 1862]. Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/24298sf8rj-cor.
Chicago
Early, John, 1786-1873, Letter from John Early (Richmond, Virginia) to Robert Paine (Aberdeen, Mississippi) - August 27, 1862, August 27, 1862, Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/24298sf8rj-cor.