MLA
Dabney, Charles W. Letter from Charles W. Dabney (Knoxville, Tennessee) to General William E. Mickle (New Orleans). Letter Refers to Dabney's Father's Book on Stonewall Jackson - June 15, 1903. 1903. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 8, Folder 18. https://digital.library.emory.edu/purl/27312jm6t3-cor.
APA, 6th edition
Dabney, C. W. (1903). Letter from Charles W. Dabney (Knoxville, Tennessee) to General William E. Mickle (New Orleans). Letter refers to Dabney's father's book on Stonewall Jackson - June 15, 1903 (Box 8, Folder 18) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/27312jm6t3-cor.
Chicago
Dabney, Charles W. “Letter from Charles W. Dabney (Knoxville, Tennessee) to General William E. Mickle (New Orleans). Letter Refers to Dabney's Father's Book on Stonewall Jackson - June 15, 1903.” Letters (correspondence). Civil War Collection, 1903. Box 8, Folder 18. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/27312jm6t3-cor.