Synopsis from Goodreads

Award-winning Astonishingly original Artistic and magical Time slides around the world so strangely ... It's not easy being Princess Ko. Her family is missing, taken to the World through cracks in the Kingdom, which were then sealed tightly behind them. Now Princess Ko is running the Kingdom, and war is looming. To help her find her family, she gathers a special group of teens, including Elliot Baranski of the Farms. He's been writing secret letters to a Girl-in-the World named Madeleine Tully - and now the Kingdom needs her help. Madeleine and Elliot must locate the missing royals, convince them of their true identities, and figure out how to unlock the dangerous cracks between the Kingdom and the World. All before their enemies can stop them.