It is Rowling's first novel since her final Potter book in 2007.
Alistair Moffat, a festival director, said: "Having one of the best-known writers in the world at Lennoxlove Book Festival is a great coup for us. We are delighted to have secured JK Rowling and to be able to offer the friends of the festival, who have supported it so loyally so far priority booking.
"Best yet is a very dangerous phrase, but I am confident in proclaiming this year's festival programme undoubtedly the best yet."
Held at Lennoxlove House, the two-day festival has 35 events for adults and children and is now in its fourth year. Authors who have previously appeared include Alistair Campbell, Kate Adie, and Sarah Brown.