That's the title for the sixth Harry Potter book.
As well as confirming that Harry Potter and the Half Blood Prince had at one point been a contender for the title of the fourth volume in the series (now Harry Potter and the Chamber of Secrets), Rowling further piqued readers' curiosity by admitting that the "half blood prince" of the title is neither Lord Voldemort or Harry himself. However, fans should not expect any further revelations; Rowling ended her message with "That's all I'm saying on THAT subject until the book's published."Well, the Chamber of Secrets is the second book - Goblet of Fire is the fourth, the one they're filming now.
I'm not crazy about that title, though. And how could it have been the fourth book? The prince must be from one of the schools that participated in the contest.
I was about to say confidently that Neville must be the Half Blood Prince...except that Neville's parents are both wizards. Oh, well. Rowing will get these last two out eventually. Even if something happens to her, the last books will be written via her detailed outline...after all, they finally got a sequel to Gone With The Wind, didn't they?