Or thinkly-veiled tome (such as an "auto-biographical novel") did the most damage to the famous person in question's reputation?
God the Father
Suri Cruise's diary.
I believe Suri's is a burn book, not a diary.
The whole world thought that Bing Crosby was a lovely man, before his son's autobiography came out. His reputation suffered the greatest change, and will not recover now that the truth is out.
Nobody ever thought that Joan Crawford and Bette Davis were anything but bitches.