MLA
Lee, Robert E. (Robert Edward), 1807-1870. Letter from Robert E. Lee (Lexington, Virginia) to James Longstreet - October 29, 1867 (photocopy). October 29, 1867. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/706g1jwtvz-cor.
APA, 6th edition
Lee, Robert E. (Robert Edward), 1807-1870. (October 29, 1867)[Letter from Robert E. Lee (Lexington, Virginia) to James Longstreet - October 29, 1867 (photocopy)]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/706g1jwtvz-cor.
Chicago
Lee, Robert E. (Robert Edward), 1807-1870, Letter from Robert E. Lee (Lexington, Virginia) to James Longstreet - October 29, 1867 (photocopy), October 29, 1867, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/706g1jwtvz-cor.