No, it's not "Book 8," but it's something fans have known for some time, which is the Harry Potter Encyclopedia she's planned on putting together.
On her website, Rowling says: "For a long time I have been promising an encyclopedia of Harry’s world, and I have started work on this now – some of it forms the new content in Pottermore. It is likely to be a time-consuming job, but when finished I shall donate all royalties to charity."
