I hate to say it, but it really depends on what the book says. A novel is by definition fiction. It can be inspired by or based on true events. The more the book is separated from the original event - such as through changing the names, locations, and identifying features of the people involved, the safer the author is. On the other hand, if I publish "The Unauthorized Biography of Fred Flinstone," and in it, I conclude that he murdered Barney Rubble with no facts and no conviction, I could be sued for libel (assuming we were talking about real people, not cartoon characters). With high profile cases or famous people, you have to be able to establish not only that the facts stated in the book are untrue, but that the author knew they were untrue or showed a reckless disregard for whether the facts are true or false. That's why unauthorized of famous people pop up - by choosing to be famous, they've given up their right to privacy. Similarly, a strong of unsolved murders could be considered a matter of public importance, so a book on that topic would also be subject to the "knowing and reckless disregard" standard.
If you're considering suing someone due to the content of a book they published, the first step would be to read it and find out what it says. Liability is going to be largely based on the content of the book.
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