I have yet to read Scott McClellan's book. I have heard him on several talk shows, most notably Fresh Air. Whenever I heard him at the WH press briefings he always came across as the man with the most thankless job in Washington. Reality was so plain to see yet McClellan had to steadfastly deflect reality based questions. He did so as well as any man could.
While his superiors could hide behind lawyers, rightwing talk show hosts, the aloofness of "executive privilage" McClellan had to be regularly fed to the dogs. Now, when he is free to exercise his right to speak his mind, they call him a traitor. The traitors calling McClellan a traitor? That is rich.
The reason the McClellan, like many other x-WH people, come out with stories and views diametrically opposed to the White House line, is only that people simply can't live in unreality forever. Once they escape they need to clear their heads from all the nonsense and spin. How else could we expect them to live with themselves? It wastes so much energy and is so exhausting keeping a lie alive. The truth, able to stand on its own, is far easier to deal with.