The Civil War Archive: The History of the American Civil War in Documents by Henry Steele Commager <http://www.amazon.com/gp/product/1579121101> contains primary source material and editor-written introductions to some sections. For the purpose of the Works Cited list, it doesn’t matter whether the publication is a photographic facsimile of the document or a transcription of the text content. This citation for a letter in the book looks the same same as the example for a published letter in MLA:

Lincoln, Abraham. Letter to E. B. Washburne. 13 Dec. 1860. The Civil War Archive: The History of the American Civil War in Documents. Ed. Henry Steele Commager and Erik Bruun. Rev. and expanded ed. New York: Black Dog and Leventhal, 2000. 44-45. Print.

If you also use the editor’s notes or introductions, create an entry under the editor’s name. To avoid redundancy you may use cross referencing.  To do that, first make a full citation for the entire anthology.

Commager, Henry Steele, and Erik Bruun, eds. The Civil War Archive: The History of the American Civil War in Documents. Rev. and expanded ed. New York: Black Dog and Leventhal, 2000. Print.

Then make abbreviated citations for the sources you used as, for example, this one for the letter:

Lincoln, Abraham. Letter to E. B. Washburne. 13 Dec. 1860. Commager and Bruun 44-45.

Your parenthetical references might read as follows:

…and secession “brought frantic efforts to compromise” (Commager and Bruun 44). Lincoln, who would not compromise in regards to slavery, wrote to Washburne, stating “blah blah blah” (45).