It's a play by Jack Thorne based on a story by Jack Thorne, John Tiffany, and JK Rowling.
So she helped come up with the story (maybe- she may just get credit since she created the characters)
But she didn't write it, Jack Thorne did.
And again, that link clearly states it's a script book. I'm sure they'll novelize it eventually, because people will buy it, but all there is right now is the printed script of the play.