My number one recommendation for people who want to get into the Civil War is In the Presence of Mine Enemies by Edward Ayers. The Civil War is probably the most written about thing in American history; there are 6,000 books about the battle of Gettysburg alone. This is at least in part because there's such a vast volume of period sources to draw on; as Ayers put it, you can find your 200 quotes and make any argument you want about the Civil War.

What Ayers does is limit the scope of his study to two counties; one in the north, one in the south. They're both in the Shenandoah Valley; they're the same peoples, the same religion, the same climate. He gathered every primary source he could find on the two counties and based his book on them; it's his way of imposing some academic self discipline, making himself prove his thesis instead of just cherrypicking. It's his way of isolating the difference between north and south, why these communities that had been so closely tied together would spend four years trying to destroy each other.

Unsurprisingly, he finds that slavery is the difference. There's an article attached to the online Valley of the Shadow database called The Difference Slavery Made that looks at this aspect specifically, but Ayers goes on to give his narration of how this difference translated into the course of events. Interestingly, despite finding this fundamental division between North and South, Ayers does not believe the conflict was inevitable, because the interests of the North and South were not in conflict. Rather, it was contingent on the decisions of the people involved, including ordinary people. And once the war came, neither reunification nor emancipation were inevitable.