MLA
Lee, Mildred Childe, 1846-1905. Letter from Mildred Lee, daughter of Robert E. Lee (West Point, Virginia) to Colonel G. N. Saussy (Hawkinsville, Georgia), thanking him for copy of his article, "Lee's Audacity" - March 17, 1902 (photocopy). March 17, 1902. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/296wwpzhp7-cor.
APA, 6th edition
Lee, Mildred Childe, 1846-1905. (March 17, 1902)[Letter from Mildred Lee, daughter of Robert E. Lee (West Point, Virginia) to Colonel G. N. Saussy (Hawkinsville, Georgia), thanking him for copy of his article, "Lee's Audacity" - March 17, 1902 (photocopy)]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/296wwpzhp7-cor.
Chicago
Lee, Mildred Childe, 1846-1905, Letter from Mildred Lee, daughter of Robert E. Lee (West Point, Virginia) to Colonel G. N. Saussy (Hawkinsville, Georgia), thanking him for copy of his article, "Lee's Audacity" - March 17, 1902 (photocopy), March 17, 1902, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/296wwpzhp7-cor.