Another nominee - but with an * - is Stephen Crane's The Red Badge of Courage. The story, for those in Europe who might not be acquainted with it, deals with a (probably teenaged) soldier, a new recruit, at the beginning of the Civil War, and how he alternates between fighting and running away in disgrace in his first battle experience.
The asterisk is for the two versions which exist now: the shorter originally published version is rather different from Crane's original manuscript, which was restored by Professor Henry Binder and published in 1979.
The restored original is to be preferred over the rather severely truncated, and ultimately incompetently edited version, which many Americans read in school. I will not be too detailed in delineating the differences, so as not to spoil anyone's experience, but one critic remarked that the book must now be reinterpreted: the war becomes much more realistic, and the main character is a much more complex figure.
So if you have not read the Binder version with Crane's original intentions intact, or have never read the book, this is the one to read.