MLA
Davis, Donnie[?]. Letter from Donnie[?] Davis to "My Dear Brother" - March 11, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 2, Folder 52. https://digital.library.emory.edu/purl/69673n5vcn-cor.
APA, 6th edition
Davis, D. (1864). Letter from Donnie[?] Davis to "My Dear Brother" - March 11, 1864 (Box 2, Folder 52) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/69673n5vcn-cor.
Chicago
Davis, Donnie[?]. “Letter from Donnie[?] Davis to ‘My Dear Brother’ - March 11, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 2, Folder 52. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/69673n5vcn-cor.