But you can only get your hands on the card if you've got about £6,000; it's being auctioned for charity and that's how much it's expected to raise.
It'll be sold at Sotheby's auction house in London on 12 December and JK will autograph it, too.
The money will go to Book Aid International which helps the world's poorest people get access to books and education.
Asked for more clues to the plot, JK's spokeswoman would only say: "I know no more than what is on that card."
The book should hopefully be ready in the shops sometime next year.