I'm not going to do a review, but a series of shorter posts about topics covered. I was familiar with over half of this information, but it is nicely connected here, and my biggest takeaways were extensions of his earlier writings. First, it is not only that we often construct elaborate rationalisations for our moral beliefs, but that we don't mind too much if they aren't first class. We are happy to settle for good-enough reasons, and don't seek better ones. Even if we encounter better arguments, we tend not to pay much attention to them, so long as we can retain our good-enough ones. This is much of the reason why minds do not change. To take my recent gun control example, there are lots of folks in favor of added gun legislation who say "well, there is a lower homicide rate in European countries, where gun laws are stricter." Attempts to show that this is so only in specialised cases, or that the low rate actually predates the legislation will be shrugged off. The good-enough reason remains intact, they need look no further.
This not only those unreasonable other people, this is all of us. Which is why we find each other infuriating. A superior argument has a hard time even winning a hearing. Not that we do not reason at all nor listen to each other. In fact, once we have made up our minds about something it is usually almost impermeable to change from our own interior reasoning. It is only significant disillusionment because of circumstances, or social persuasion by others.
It is not that reasoning does not affect us, but that it is limited to feedback from others, and tends to be gradual. Haidt use the image of an elephant and rider. The intuitive part of our moral reasoning, the elephant, is 90% of the picture. The rational part, the rider, is but 10%. (He has some demonstration that this is a good thing and not to be despised.) The rider can steer events, and can ultimately direct where the elephant will go, but it is difficult and involves constant recalculation as the elephant does a whole lot of just-as-he-pleases.
I recognised one of my own major decisions in Haidt's context. I retained doubts about whether believing Christianity was intellectually defensible when I first converted (or returned to the faith, depending on perspective). But I trusted that CS Lewis had encountered many more of the philosophical arguments than I had - having taken a First and become a philosophy professor in the early part of his career - and was far smarter and more familiar with the big issues in Western thought. I took this as assurance that theism, and especially Christianity was at least not entirely in tatters and could be provisionally accepted. You will note that this was merely a good-enough reason, not any airtight case. That sufficed for a long time.