Killer Angels would be a good one. Here's my top 5 (the order changes from time to time, but they're all great):
1. The Killer Angels -- Absolutely best Civil War Novel. Red Badge of Courage is fairly good for an opium addict who was about 3 when the war actually took place. But K.A. kick's its ass.
2. The Winds of War -- The absolute best World War II novel, and the only epic novel I've read three times. You learn the history real well reading it.
3. August, 1914 - Alexander Solszhenitsyn -- Skip the first five chapters (which I think he put in to get past the Soviet censors). After that, this is an awesome novel.
4. All Quiet on the Western Front -- This is a classic, and a little cliched now, but it created the cliches, so that's alright. It's anti-war, but what would you expect from a German writer with a French last name?
5. A Farewell to Arms -- Hemmingway's first major work, and one of his best (probably second only to "A Sun Also Rises."
. This is more novel than history, and more literature than simply fictionlizing battles, but it is an awesome book and should be read by everyone. Also a bit anti-war, but again, this was WWI, which was historically the most pointless major war.
Although I'm a huge Texas history buff, I have found no great Texas novels. "Texas" is alright, but it's long and preachy, and Mitchener (who is normally a great writer) tries to be too encyclopedic. He's trying to get all of Texas history in the book, when what makes a great novel great is that it is selective. While the Spanish colonial period is incredibly important, there's just no way to make it exciting in narrative unless you write it from the perspective of comanche indians who captured a priest and are flaying him alive.
And NOBODY has ever done a good novel about things like the Mier Expedition which has a potential scene (the counting of the beans) which has enough potential drama to make Hitchcock hide under the couch.