I haven't read the book yet, so I can't comment on it. Nor have I had a chance to see Clarke's complete testimony (although I have it on tape, and will watch it at the first opportunity I get.)
As for the White House not having the power to censor Clarke's book, I don't think it's a question of censorship so much as a responsibility to the truth. Let's say, just for arguments sake, that Clarke's book is inaccurate. If you were one of the people who were in charge of approving this book, and you caught a significant inaccurate fact, how would you handle it? Would you want to bring the error to light, and offer evidence that would allow Clarke to correct the mistake? Or would you let it go as is? People are going to be reading this book believing it to be the truth (some of them, anyway), and you have an opportunity to make sure that what they're reading really is the actual truth. Wouldn't you feel a responsibility to the truth to make sure that the readers get the whole story?