When reading the excerpt it certainly reads as credible and doesn't point to anything unreasonable. I can understand the doubt people have but something about it feels authentic. Of course it could be one mans elaborate effort to create a book with some ulterior motive. One thing for sure is that if the book attempts to poo-poo BTC and praise BCH we'll know instantly that it's a load of bullshit. But nothing in the excerpt seems to suggest that. If what he's going to set straight is that 'Craig Wright isn't Satoshi' then that would be a big credibility bonus. There could also be many reasons for not signing with his own key. Perhaps as part of his big plan to disappear was also to destroy his keys thereby ensuring that he can never spend the first minted coins or authenticate himself in the future. To ensure that S. Nakamoto dies when his key dies so that not even himself can go back on his grand plan.
For now, it was a great read and certainly seemed to come from the heart. Especially liked the mention of Hal Finney. A true legend in the Bitcoin Universe.