MLA
Thornton, Felix B. Letter from Felix B. Thornton (Williamsburg, Virginia) Who Served in Company C of the 53rd Georgia Infantry Regiment. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 53. https://digital.library.emory.edu/purl/989r22815r-cor.
APA, 6th edition
Thornton, F. B. Letter from Felix B. Thornton (Williamsburg, Virginia) who served in Company C of the 53rd Georgia Infantry Regiment (Box 6, Folder 53) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/989r22815r-cor.
Chicago
Thornton, Felix B. “Letter from Felix B. Thornton (Williamsburg, Virginia) Who Served in Company C of the 53rd Georgia Infantry Regiment.” Letters (correspondence). Civil War Collection, n.d. Box 6, Folder 53. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/989r22815r-cor.