MLA
Gilmore, J. O. Letter from J. O. Gilmore, Assistant Adjunct General (Washington, D.C.) to William E. Mickle Jr. (Mobile, Alabama) Regarding William's Proposed Commission to the United States Army - July 29, 1899. 1899. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 9, Folder 17. https://digital.library.emory.edu/purl/24298sf8p0-cor.
APA, 6th edition
Gilmore, J. O. (1899). Letter from J. O. Gilmore, Assistant Adjunct General (Washington, D.C.) to William E. Mickle Jr. (Mobile, Alabama) regarding William's proposed commission to the United States Army - July 29, 1899 (Box 9, Folder 17) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/24298sf8p0-cor.
Chicago
Gilmore, J. O. “Letter from J. O. Gilmore, Assistant Adjunct General (Washington, D.C.) to William E. Mickle Jr. (Mobile, Alabama) Regarding William's Proposed Commission to the United States Army - July 29, 1899.” Letters (correspondence). Civil War Collection, 1899. Box 9, Folder 17. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/24298sf8p0-cor.