Jake Ransom and the Skull King's Shadow (Jake Ransom No. 1)

When a mysterious envelope arrives for Jake Ransom, he and his older sister, Kady, are plunged into a gripping chain of events. An artifact found by their parents—on the expedition from which they never returned—leads Jake and Kady to a strange world inhabited by a peculiar mix of long-lost civilizations, a world that may hold the key to their parents' disappearance.