(FORTUNE Magazine) – Booksellers around the world have been aching to get their hands on Harry Potter and the Half-Blood Prince, which finally hits stores this month. After all, the five books currently in print have sold a total of 270 million copies worldwide, and Scholastic's first run of book six sets an all-time record at 10.8 million (fans, of course, are just dying to know whether Ron and Hermione finally hook up). But how much staying power will this Potter have? While the first three installments continued to sell millions of copies for years after their releases, books four and five started out huge, then saw a precipitous drop-off in sales until paperbacks were issued (after selling 7.9 million copies when it was released in 2000, Goblet of Fire sold a mere 295,000 copies in 2001). Scholastic's Barbara Marcus isn't worried. "New readers come to Harry Potter as they reach the age of 8 or 9," she says. "The books will be read for generations to come." Here's a look at how Potter's magic has worked over the years. -- Kate Bonamici