MLA
Reep, Rueben Beale. Letter from Reuben Beale Reep (Camp near Gallatin, Tennessee) to "Dear Friends". Reep Was a Member of the 105th Ohio Infantry Regiment, Company A, and Entered Service on August 1862 and Died in a Field Hospital in Cowan, Tennessee on July or August, 1863 - December 21, 1862 (Typescript). 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 20, Folder 17. https://digital.library.emory.edu/purl/7032280hgm-cor.
APA, 6th edition
Reep, R. B. (1862). Letter from Reuben Beale Reep (Camp near Gallatin, Tennessee) to "Dear friends". Reep was a member of the 105th Ohio Infantry Regiment, Company A, and entered service on August 1862 and died in a field hospital in Cowan, Tennessee on July or August, 1863 - December 21, 1862 (typescript) (Box 20, Folder 17) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/7032280hgm-cor.
Chicago
Reep, Rueben Beale. “Letter from Reuben Beale Reep (Camp near Gallatin, Tennessee) to ‘Dear Friends’. Reep Was a Member of the 105th Ohio Infantry Regiment, Company A, and Entered Service on August 1862 and Died in a Field Hospital in Cowan, Tennessee on July or August, 1863 - December 21, 1862 (Typescript).” Letters (correspondence). Civil War Collection, 1862. Box 20, Folder 17. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/7032280hgm-cor.