Like if I want to write a book series about Harry Potter character Rita Skeeter's life, may I do it? I always hear that this isn't allowed. But as far as I know some people have written a book about what happened after Potter won the war. And the series "män som hatar kvinnor" has a fourth book called "Det som inte dödar oss".
So is it allowed to write a book (and make money with it), if you base the plot and/or the characters on something written in another book?