MLA
Haskell, W. W. Letter from W. W. Haskell (Camp Gregg. Virginia) to Madam Goettle. Haskell, Who Served in the 12th Massachusetts Infantry Regiment, Company K, Discusses the Death of Madam Goettle's Son - March 23, 1863. 1863. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 33. https://digital.library.emory.edu/purl/358bg79dr9-cor.
APA, 6th edition
Haskell, W. W. (1863). Letter from W. W. Haskell (Camp Gregg. Virginia) to Madam Goettle. Haskell, who served in the 12th Massachusetts Infantry Regiment, Company K, discusses the death of Madam Goettle's son - March 23, 1863 (Box 19, Folder 33) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/358bg79dr9-cor.
Chicago
Haskell, W. W. “Letter from W. W. Haskell (Camp Gregg. Virginia) to Madam Goettle. Haskell, Who Served in the 12th Massachusetts Infantry Regiment, Company K, Discusses the Death of Madam Goettle's Son - March 23, 1863.” Letters (correspondence). Civil War Collection, 1863. Box 19, Folder 33. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/358bg79dr9-cor.