MLA
Eustis, F. A. Letter from F. A. Eustis (Newport, Rhode Island) to "My Dear Rosa" Regarding the Settlement of His Mother's Estate Right at the Conclusion of the Civil War - August 5 [1866?]. 1866. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 8, Folder 22. https://digital.library.emory.edu/purl/334fn2z3w1-cor.
APA, 6th edition
Eustis, F. A. (1866). Letter from F. A. Eustis (Newport, Rhode Island) to "My Dear Rosa" regarding the settlement of his mother's estate right at the conclusion of the Civil War - August 5 [1866?] (Box 8, Folder 22) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/334fn2z3w1-cor.
Chicago
Eustis, F. A. “Letter from F. A. Eustis (Newport, Rhode Island) to ‘My Dear Rosa’ Regarding the Settlement of His Mother's Estate Right at the Conclusion of the Civil War - August 5 [1866?].” Letters (correspondence). Civil War Collection, 1866. Box 8, Folder 22. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/334fn2z3w1-cor.