The Master of Shadows (Elsewhere Chronicles)

Rebecca, Max, Noah, and Theo are reunited and on the run. The Master of Shadows himself is close behind. Nowhere is safe - monsters lurk in the water and on land, and the Shadow Spies strike at nightfall. All they have to guide them are the strange clues left by Grandpa Gabe. Can the four of them untangle Grandpa Gabe's secrets in time? What will it cost them to stop the Shadows?