MLA
Fowler, Thomas. Letter from Thomas Fowler (Camp near Vicksburg, Mississippi) to His Wife, Rebeckah Fowler - April 3, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 3, Folder 28. https://digital.library.emory.edu/purl/385n8pk1rs-cor.
APA, 6th edition
Fowler, T. (1863). Letter from Thomas Fowler (camp near Vicksburg, Mississippi) to his wife, Rebeckah Fowler - April 3, 1863 (Box 3, Folder 28) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/385n8pk1rs-cor.
Chicago
Fowler, Thomas. “Letter from Thomas Fowler (Camp near Vicksburg, Mississippi) to His Wife, Rebeckah Fowler - April 3, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 3, Folder 28. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/385n8pk1rs-cor.