I think you misunderstand the author’s job here. This is a book that is, in some of the passages that have been printed, sets out what other people have told him happened at various points in time. He is recording their firsthand accounts, not himself establishing what was true or not. It’s not a judgement.
Both you and bower don't seem to misunderstand what writing a biography or journalism is. It's not repeating a bunch of lies, without checking. It is your duty to check.Especially when something is so well publicised, you should be aware of it.That's the danger of writing about subjects you hate, you know nothing about them.
Just like Bower himself was verified as a liar by another journalist, who did his job of 'fact checking' and showing where all his lies are. Otherwise what is the point of telling the public lies in the form of a book.
Do not confuse this with the Oprah interview, which we were already pre-warned was their story from their mouths, not a biography. Unscripted, on the spot.
Even Oprah had some questions in her general knowledge, where she asks MM about what was being reported about her.