He did not write the book to defeat Bush.
He wrote the book to cover his ass.
He knows that to refute many of his implications-by-omission, that the President cannot reveal various secrets, especially operational secrets.
He wrote the book to cover his ass.
I think you may be onto something there. That might explain why he has such a vested interest in it. That might also imply he's been assigned to cover up for others who were complicit. . .