This event is roughly equivalent to discovering that your next-door neighbor of 20+ years has a ten year-old child at home who has never been mentioned. And a rather brilliant kid at that.
She was simply writing along on this never-really-meant-for-publication project. And now it blurps forth in full sharp glory. I may not have mentioned: it's an exceptionally bold book as well.
Telling oneself that a project is just for fun or private writing is an excellent disinhibitor and freer of creativity.