This is a preliminary impression, of course. I haven't read the book yet, though there's a good chance I'll have a copy tomorrow. But I watched the Scott McClellan interview with Keith Olberman tonight, and he seemed pretty credible, in the sense that he seemed to stick to what he had reason to know about and to resist speculating on what he didn't or couldn't know about. I'll have more to say later.
I still think he wasn't much of a press secretary, especially in the sense that he wasn't especially good at communicating what the administration was trying to do. I guess I'm not surprised it took him so long to get disillusioned; loyalists can often overlook vague and growing uneasiness for a long time. My preliminary impression, however, is that he did the book for what he conceives to be patriotic motives. The White House, and Ari Fleischer, of course, are trying to say he didn't have enough access to say what he has said, but of course nobody will admit to having read the book yet. So far I haven't seen a convincing refutation of any of the facts or incidents he's written about.
If he's right that Bush's impulses from an early date were more Wilsonian than defensive -- that he wanted to attack Iraq to impose "coercive democracy" (nice turn of phrase) more than because the intelligence was so supposedly airtight on WMD, it's an important thing to know. It would mean that Bush is an even bigger fool than we thought.