Yeah, I have to say Killer Angels as well. It's a cliche, but it is probably one of the best-written historical novels ever.
Gods and Generals is written by the K.A. author's son, and is in a similar style. It is good but not great. Better book than a movie, but mostly because the movie followed the book TOO closely, and hence was long and ponderous.
The Red Badge of Courage is a fascinating book, but was written by a drunken, opium-addicted college punk 20 years after the last shot had been fired. So it's not really up there on authenticity. Still, considering his limitations, Crane didn't do a bad job.
Outside of those, I have to admit not having read too much Civil War nonfiction.