We only have the Rabbi's word on that. He showed no paperwork or contract to show that he even had Michael's permission. The only ones who know is Michael and the Rabbi. I highly doubt he is telling the truth about that audio.
Michael recorded everything, but that doesn't mean he wanted to share it with the world. Like those home movies of his children that were leaked. That was for his private viewing, not splatter all over the news.
Even if a book was to be made from those tapes, Michael is no longer alive to edit it. We only have the Rabbi who was nothing of a friend to Michael. Rabbi knows what he is doing would had gotten him sued since he waited until Michael was dead to show these tapes.
Although Michael himself said nothing really new, he went into far more detail than what he even did with Bashir. Which is why I doubted he ever wanted them release. Specially about the abuse Joe gave him and even his comments about Madonna, since Michael was very careful about talking about other people. Especially, fellow artists.
So to to me, that book and those tapes are a complete invasion of privacy. When I die, I wouldn't want people putting my private messages for the world to see and I think Michael would feel the same way.
