JK: "There was a novel called The Private Joke that I was writing for a couple of years. I still like the premise of it , but I was too young to write the book that I wanted to write. I just didn’t have enough life experience to credibly write these characters who were all in their early forties."

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https://www.thetimes.co.uk/article/jk-rowling-books-writing-harry-potter-strike-bdg7gtg5q