MLA
Thruston, Gates Phillips, 1835-1912. Letter from Philip Gates (New York) to H. Y. McCord (Atlanta, Georgia), written at the time of Gates' sending the letter to Mr. McCord - February 23, 1899. February 23, 1899. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/1741rn8q8r-cor.
APA, 6th edition
Thruston, Gates Phillips, 1835-1912. (February 23, 1899)[Letter from Philip Gates (New York) to H. Y. McCord (Atlanta, Georgia), written at the time of Gates' sending the letter to Mr. McCord - February 23, 1899]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/1741rn8q8r-cor.
Chicago
Thruston, Gates Phillips, 1835-1912, Letter from Philip Gates (New York) to H. Y. McCord (Atlanta, Georgia), written at the time of Gates' sending the letter to Mr. McCord - February 23, 1899, February 23, 1899, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/1741rn8q8r-cor.