Rowling spent seventeen years writing the seven Harry Potter books. In a 2000 interview through Scholastic, her American publisher, Rowling stated that there is not a university after Hogwarts and "I won't say "never," but I have no plans to write an eighth book." However, she states in her website that she might write an eighth book for charity.
When asked about writing other books similar to Quidditch Through the Ages and Fantastic Beasts and Where to Find Them, she has said that she might consider doing this with proceeds donated to charity, as was the case with those two books. Another suggestion is an encyclopedia-style tome containing information that never made it into the series, also for charity.
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