That book appears to be written in the way that B and C list 'celebrities' here in the US write their memoirs; all shock, scandal, and bullshit to try to boost their careers through publicity. As PT Barnum said "Any publicity is good publicity", or words to that effect.
Add in Harry's need to justify himself to himself (and the world) by slating his family and airing dirty laundry and you have the perfect storm. What Harry forgot about airing dirty laundry is that your own is also hanging out in plain view for all to see. And right now the focus appears to (rightly) be on his own dirty pants flapping in the breeze.
Where's Meghan in all of this? I think they have a pretty f'd up relationship dynamic to begin with. But IMHO, she's encouraging him, adding to his grievances, justifying his actions to him. Publicity for him is publicity for her, and again, anything that gets her name in the papers works for her. She knows they have a 'brand' to keep in the public eye. And 'brands' take money, lots of money.
I am not saying that she and he may not have some valid grievances. In any family there are problems. But the right way to handle them is not to blow that family apart and then sell the explosion for millions. Nor to exact petty revenge for past problems by revealing things best kept private.