The evidence is pretty clear Scott McClellan was lying through his teeth when he was White House Press Secretary, why the heck should I believe he's telling the truth now as he peddles his book? The main difference between him and Fleischer and Snow is that he wasn't that good at the lying. Perino is about as bad as he is.
All these books - McClellan, Fleischer, Bremer, Feith - are uniform in their spin that poor Bush was done in by his advisors. The truth is the buck stops at the president's desk.