All Quiet on the Western Front by Remarque is very good.
I like Hemingway's war novels too, A Farewell to Arms, For Whom the Bell Tolls and Islands in the Stream, and his stories of the Spanish Civil War in The Fifth Column.
Two others that come to mind are A Separate Peace and Red Sky at Morning.
There's an interesting historical novel by Elmore Leonard titled Cuba Libre, about the Spanish-American War.
And don't forget Vonnegut-- Slaughterhouse-Five. "Billy Pilgrim has come unstuck in time."