Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Respectful, Sympathetic
Shaara is respectful toward his subject. He doesn't heap any hero-worship at the feet of General Lee, for example, but he acknowledges the real heroism that characterized the battle, for instance in Chamberlain's bayonet charge. Also, Shaara's omniscient perspective and ability to enter the heads of different characters make his tone more sympathetic. He doesn't portray the Confederates as one-dimensional villains; instead, he strives to enter into their perspective, even when he doesn't agree with it. Since he places you right in the characters' heads, the writing, even though it's dealing with major historical events, has an up-close and intimate feel to it.