To be considered nonfiction, a book needs to be completely true, although there are some allowances that are allowed to be changed, like dialogue as long as it’s close to the original. Also, I don’t need to know every single detail about what was happening. I’d love to hear the moment you fell in love, but please do not tell me that you wore a green turtleneck, dark washed denim jeans, and the diamond necklace that your grandmother gave to you after she died of pneumonia when you were eighteen.
Half-truths and three-quarter truths aren’t acceptable, but something as close to 100% is a good thing. If it’s 100%, bonus points for the book. For me, I’d say the acceptable range is probably 95-100% true is a good memoir. If you’re telling the book over a long period span of time and it starts when you were born, of course there might be a little discrepancy of the truth and what you wrote. But also, don’t embellish a detail that people have a record of. If you got arrested for something, don’t build it up to be something that it wasn’t. If you steal a candy bar and go to jail for an hour then pay the fine, do not say that you shot someone and blacked out and forgot what happened but as you are about to be killed by the death penalty have a sudden revelation that you killed the person in self-defense and the proceedings get halted and they figure out that yes, what you said is true. That’s an embellishment I can’t deal with. But going back to stealing the candy bar, if you say you were in jail for two hours when it was really an hour and a half, I’m fine with the time difference. It’s almost like the same thing right?
I think that we do need genre lines. There are some books where you can definitely tell that it’s a fiction book because it’s never going to happen in real life (think teenage girl falling in love with a vampire here . . .), but then there are some books where they are proven and credible facts that this happened and everything the author says is 100% true. That’s when you know it’s a nonfiction work.