The movie script for Gettysburg relies heavily on Michael Shaara's Pulitzer-prize winning novel, The Killer Angels. Gettysburg has its ups and downs, but the novel is the basis for most of my complaints about historical interpretation in the film. Also, in my opinion, the movie doesn't treat the motivation soldiers of either side had to be at war with one another.

My wife enjoyed Col. Fremantle's diary, the first book about the war she ever read:

http://www.colfremantle.com/journey.html