I've been in the midst of a ton of research on the American Civil War, because my fourth novel is set there. Although I didn't spend most of my time looking deeply into the causes of the Civil War, I have by now read my fair share about it. I read one of the seminal books on the Civil War, called “Battle Cry of Freedom” by James McPherson. I'd recommend it to anyone. It has a really nice set of chapters that lays the foundations for the war.