From the Associated Press:
A rumormonger has jarred this secret from superstar author J.K. Rowling: Book six, currently being written, will be titled “Harry Potter and the Half-Blood Prince.”
Rowling says that’s “the title I have been using in my head for ages and ages” and that she considered it for book four, later called “Harry Potter and the Chamber of Secrets.”
Rowling tipped her hand obliquely after a fan posted a rumor claiming that the upcoming book would be titled “The Pillar of Storg�” That, Rowling replied on her official site (www.jkrowling.com), “was never my title and I did not change it at the eleventh hour because I was ‘found out.’ ”
Publisher Scholastic confirms the new title but says you’ll have to “search for clues” to find it on her site. Rowling further revealed that the half-blood prince is neither Harry nor Lord Voldemort, but “that is all I’m saying on THAT subject until the book’s published.”