Writing on website Pottermore, Rowling also details how she came about naming Uncle Vernon and Aunt Petunia.
Another million more copies being printed, says publisher.
Novel is fourth in the erotic franchise and was released on Christian Grey's birthday 18 June.
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Solo was depicted as married in the latest issue, taking place between A New Hope and The Empire Strikes Back.
Grey will be released on 18 June, Christian Grey's fictional birthday
Harry Potter-themed live action role play (Larp) cast its spell over more than a hundred would-be sorcerers.