MLA
Kelley, Thomas F. Letter from Thomas F. Kelley (Camp near Petersburg, Virginia) to Charles Bailey Regarding the Death of His Son, Austin F. Bailey. 1865. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 2. https://digital.library.emory.edu/purl/334fn2z48p-cor.
APA, 6th edition
Kelley, T. F. (1865). Letter from Thomas F. Kelley (Camp near Petersburg, Virginia) to Charles Bailey regarding the death of his son, Austin F. Bailey (Box 19, Folder 2) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/334fn2z48p-cor.
Chicago
Kelley, Thomas F. “Letter from Thomas F. Kelley (Camp near Petersburg, Virginia) to Charles Bailey Regarding the Death of His Son, Austin F. Bailey.” Letters (correspondence). Civil War Collection, 1865. Box 19, Folder 2. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/334fn2z48p-cor.