* No Public Release of Sales Data: Publishers and authors don't always release precise sales figures.
* "Harry Potter and the Prisoner of Azkaban" was released in 1999: This was the third book in the series, and sales would have been influenced by the popularity of the first two books.
* International Sales: Rowling's books were selling worldwide by 1999, making it even harder to track exact numbers.
However, we can say that 1999 was a very significant year for Rowling's sales. "Harry Potter and the Prisoner of Azkaban" became a massive bestseller, solidifying the series' place in popular culture.