MLA
Barr, Wolcott S. Letter from Wolcott S. Barr (Falmouth, Virginia) to His Brother, Eugene Written on April 26. Within the Letter to His Brother Is a Letter to His Mother, Elmira Barr Written on April 28. Wolcott S. Barr Served in Company I of the New York 80th Volunteer Infantry Regiment - April 26-28, 1863. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 3. https://digital.library.emory.edu/purl/9289ghx4m9-cor.
APA, 6th edition
Barr, W. S. (1864). Letter from Wolcott S. Barr (Falmouth, Virginia) to his brother, Eugene written on April 26. Within the letter to his brother is a letter to his mother, Elmira Barr written on April 28. Wolcott S. Barr served in Company I of the New York 80th Volunteer Infantry Regiment - April 26-28, 1863 (Box 19, Folder 3) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/9289ghx4m9-cor.
Chicago
Barr, Wolcott S. “Letter from Wolcott S. Barr (Falmouth, Virginia) to His Brother, Eugene Written on April 26. Within the Letter to His Brother Is a Letter to His Mother, Elmira Barr Written on April 28. Wolcott S. Barr Served in Company I of the New York 80th Volunteer Infantry Regiment - April 26-28, 1863.” Letters (correspondence). Civil War Collection, 1864. Box 19, Folder 3. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/9289ghx4m9-cor.