"From the Fields of Fire and Glory: Letters of the Civil War" by Rod Gragg. Interesting because it includes reproductions of the actual letters.
"All for the Union: Civil War Diary and Letters of Elisha Hunt Rhodes" by Robert Hunt Rhodes. Union soldier prominently quoted in Burns' documentary.
"For Cause and Comrades: Why Men Fought in the Civil War" by Dr. James McPherson.
"Mother May You Never See The Sights I Have Seen: The 57th Massachusetts Veteran Volunteers in the Last Year of the Civil War" by Warren Wilkinson.