To get the best understanding, start not with books about the Civil War, but read the books about the aprox 10 years prior to from about 1850 to 1860. Then you will have a better understanding how.
The nitty gritty details of the war itself are well documented. But the decade before will give you more insight into the why. There are things pro and con regarding secession, slavery and a variety of other issues that folk on both sides found to be an egregious violation of their...insert appropriate here (religion, rights, morality, etc.)
Once I had a better understanding of the whys on both sides, the mechanics of how came into a much clearer focus. Many of the Civil War books try to address why, but that is just the author’s opinion. Read the history about the before, and form your own opinion on it and re-read what is out there about the war. It is most illuminating.
That’s why I recommend the first book in the Bruce Catton trilogy: The Coming Fury. It covers the period before the Civil War.