"Ladbrokes has reopened its "will there be an eighth Harry Potter book" market, following a number of requests from fans. The betting firm is offering odds of 2/1 on JK Rowling releasing another installment of the phenomenon by the end of 2012. It is offering 1/3 on there not being another Potter novel." (From the Bookseller blog.)
Apparently, there was quite a flurry of bets immediately after publication of Harry Potter and the Deathly Hallows (book 7), then all went quiet. Now, people are wanting to bet again. Well, a fool and his (or her) money are easily parted. It is possible, I suppose, that this recent activity has been stimulated by the current New York court case, in which J. K. Rowling is trying to stop someone from plagiarising chunks of her books into a self-styled "lexicon". Be that as it may, there will be no eighth Harry Potter novel. The author has long said that she might write an encyclopaedia-style book, in part to add some details. Such a volume might be based in part on the huge "style bible" that the Bloomsbury sub (copy) editors have gradually accumulated. J. K. Rowling has also occasionally mused on revising the existing novels. But there won't be a new one.