I’m just genuinely curious as to why he makes people so inflamed
I think it's generally the amount of unkindness he's shown to multiple people.
(I haven't read the book either but have read excerpts here and elsewhere.)
Examples of the top of my head
In the Oprah interview: in allowing the suggestion of racism in the family, which he later retracted in the Tom Bradby interview (which of course far fewer people will have seen); in allowing the mocking of curtseying to the late Queen.
In the book: in mocking the physical imperfections of the matron; in commenting on the person with whom he lost his virginity; in describing his brother's private parts; in describing the (private) disagreement he had with his father when the late Queen died and his father did not wish Meghan to travel to Balmoral.
This is obviously without the other issues around bullying in the Sussexes office, and the obfuscation in interviews about security.