I haven't seen the new book, but an older printing claimed that Jason killed Nicole and her boyfriend after arriving at the house and seeing them together. Jason is(was) a chef, and had just received back his work knives from a professional sharpener.
OJ covered up for Jason to spare his son.
Dear may be getting old, and pushed through a book to capitalize on his previous fame, in his retirement.
FWIW, we used to live down the road from him and he was a really nice guy, not at all a kook.
I think he is right on the money. I thought Jason did it very early on.