I look at it this way. Yes, Cynthia was there, and she'd probably know a lot more about the events in her book than any of us ever could. But what she's writing about are her impressions, which may or may not be the way it really was.
Cynthia might have written Aunt Mimi a certain way because that's the way Cyn perceived her to be. It's pretty well documented that Mimi didn't like Cyn too much, so this may be the only side of her that Cyn ever got to see. Another person might have perceived Mimi completely differently. So would either one of these people be wrong? Of course not. That's why I think you need more than one person's perspective to form your own picture of things. (Just a what-if scenario, FYI--I don't know if any of this is the case.)
I'm looking forward to this book...I'd like to hear things from Cyn's perspective and she certainly has a right to be heard. But I'm treating this like I treat all the other books about John--it's not the gospel truth. Just another piece in the puzzle that makes up the man.
"They're just a passing phase, symptoms of the uncertainty of the times and the confusion about us. I hope, when they get older, they'll get a haircut." -- Billy Graham, on the Beatles