How can we all be sure that Ray is right about what he wrote?
I don't think we can be sure. He has some pretty impressive-looking documents in the book that strike me as genuine. He certainly was on the inside, that he was a GB member is not in question. And there are those that know him that can confirm some of what he said. But to be truly "sure"? I don't think it's possible.
On the other hand, though, I think his testimony needs to be viewed in the light of other things we do know about the Watchtower. We can confirm that they have intentionally misled their members. We can confirm that they intentionally mislead the public. We can confirm that their doctrines are not all bible-based.
Given those things we ARE sure about, I can at least believe most of what Ray wrote. And in the end, if it turned out to be nothing but lies, I would still reject the Watchtower based on everything else I know about them. (Whether I was Bible-believer or not.)
Dave