So prior to reading the book, I thought of the Civil War as a morality tale: the North wants to end slavery, the South wants to continue it, and they fight a war over this.

After finishing the book, I still think it is a morality tale in the sense that the war had clear good guys and bad guys. In fact, I was struck by the heroism of those fighting for the North, and I wish we would do more to celebrate that heroism in addition to emphasizing how immoral slavery was.

However I also realized that my preconceptions were somewhat incorrect. Prior to the Civil War, a typical Northerner was "antislavery" without being abolitionist. Being "antislavery" meant you were against returning fugitive slaves to the South, and you thought slavery should be illegal in newly added states. A typical Northerner didn't like slavery, but thought it was going to die out on its own, and wanted to preserve the Union. Abolitionism was considered extreme.

Fugitive slaves generated a lot of headlines, but their absolute numbers were fairly tiny. You can probably think of some analogous modern-day political issues, where people get really worked up about something that has a fairly low impact in relative terms.

However, the newly added states thing was more important because it involved political control over Congress, the Presidency, etc. My impression from reading the book was that many Northerners were willing to seek compromise on this issue, but the Southerners were very pugnacious and kept demanding more and more slavery states, repeatedly threatening to secede if they didn't get their way -- to the point where it practically became a meme.

I think this is where the "different economic systems" thing becomes important. Slavery states and non-slavery states had very different economies (e.g. agriculture vs manufacturing), which lead to different economic interests, which is why they had consistent policy differences and control of the Congress/Presidency was important. (I'm extrapolating a bit in this paragraph -- the book doesn't explicitly put things together the way I did. However she does mention "the growing apart due to different economic systems" point from my previous comment. That wasn't something I invented myself.)

Eventually Northerners become radicalized because they can see that slavery is not ending on its own as they had hoped, because the South keeps asking for more and more slavery states, and also because of Uncle Tom's Cabin. In those days, transportation and communication were far more limited, so many in the North just didn't have much information about what slavery actually looked like as it was practiced in the South. (One of the big Republican figures of Goodwin's book, William Henry Seward, becomes a major anti-slavery activist after taking a vacation to the South with his wife. Even so, IIRC he doesn't self-identify as an abolitionist, but political opponents try to paint him as one in order to make him unelectable.)

Even so, at the crack of the Civil War, the cause that the North was officially fighting for was preservation of the Union. In those days, the US was far more self-conscious about its status as a democracy in a world where almost all known countries were still running on monarchy/feudalism. The feeling in the North was that the US had to show the world that democracy was viable, and successful Southern succession would prove that the snobby aristocrats back in Europe were right all along. In the Gettysburg Address, Lincoln doesn't say anything about the inhumanity of slavery, but he does end on this note: "... that government of the people, by the people, for the people, shall not perish from the earth." (emphasis mine)

It was only a while after the start of the war that people in the North became sufficiently radicalized that the Emancipation Proclamation became a politically viable move for Lincoln -- and even then it only applied in rebel states. Lincoln played an interesting role as a pragmatist humanitarian -- the subtext I got from the book was that he was always an abolitionist, keeping his cards close to his chest, and trying to help things along without ruining his political career. Obama's eventual support of gay marriage could be a good modern analogy here. What made Lincoln remarkable was his simultaneous concern for Black people (e.g. he once told Frederick Douglass that he valued Douglass' opinion more than any other American) and also his ability to empathize with Southern slaveholders (e.g. I think he once said something like if he'd been born a slaveholder, he would support slavery too). I found Lincoln inspiring because that sort of pragmatic, perspective-taking humanitarianism really appeals to me.