MLA
Olmstead, Charles H. Letter from Charles H. Olmstead, Joshua C. Bruyn's Commander, to K. M. Read, Expressing Thanks for Lending Him the Diary of Joshua C. Bruyn - August 6, 1921. 1921. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 1, Folder 41. https://digital.library.emory.edu/purl/3009w0vv4s-cor.
APA, 6th edition
Olmstead, C. H. (1921). Letter from Charles H. Olmstead, Joshua C. Bruyn's commander, to K. M. Read, expressing thanks for lending him the diary of Joshua C. Bruyn - August 6, 1921 (Box 1, Folder 41) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3009w0vv4s-cor.
Chicago
Olmstead, Charles H. “Letter from Charles H. Olmstead, Joshua C. Bruyn's Commander, to K. M. Read, Expressing Thanks for Lending Him the Diary of Joshua C. Bruyn - August 6, 1921.” Letters (correspondence). Civil War Collection, 1921. Box 1, Folder 41. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3009w0vv4s-cor.