People can contractually waive their rights to sue, so a waiver could protect you, in the event you could get someone to sign it. However, a key issue, if the person ever tried to litigate notwithstanding the waiver, would be whether his signing was effective under the circumstances. If you did not accurately disclose to him what the contents of the book and its representation of him would be, or worse, actively lead him to believe that it would be neutral at worst or even favorable to him--and then it wasn't--he may be able to void the waiver on the grounds it was not a knowing waiver of his rights (and/or that you committed fraud to get him to sign). Even if you can't disclose all the particulars of what you'd write, you need to be upfront that you are not promising to depict the person in any particular way and that the depiction might be one he finds unflattering--and make sure the waiver itself says that, too.