I haven't read the book, only a few unofficial excerpts from the Spanish version as reported by the press, and I haven't seen the other interviews. But based on what he said in the US inverview (60 Minutes, with Anderson Cooper) last night, this is the core of the issue:
He has a deep-seated resentment of (almost a sworn enmity against) the UK press as an institution. He blames them for blighting his life and that of others in the RF, and in part for what happened to his mother. He was afraid that something terrible would happen to his wife or child(ren) because of the toxic and hateful atmosphere fostered, and that is why he felt he had to leave the UK.
He is angry with members of his family for in his view doing some combination of directly and/or indirectly (1) encouraging/collaborating with the press (2) leaking inflammatory material to the press (3) accepting the press's narrative rather than talking directly with Harry about things such as his relationship with Meghan and/or (4) failing to speak out about how damaging and wrong the situation was. At times, it seemed that he believed the influence of the press had actually turned his own family against him. He feels particularly betrayed by others in the RF who he knows have seen the toxic effects of the press and once upon a time resented them as much as he did - assume this is William, and to a degree Charles, Kate, and Camilla.
Now the only option left to him is to use the press to fight the press. Or something like that.
(I am not endorsing his world view, just summarising what he seemed to be trying to say with all the fluff and filler removed.)