The Eye of the Forest (Children of the Lamp Series #5) by P. B. Kerr
When a collection of Incan artifacts goes missing, the Blue Djinn of Babylon dispatches the twins and Uncle Nimrod to recover them. Along the way, though, John and Philippa encounter their friend Dybbuk, who was drained of his djinn powers but is determined to get them back. In a fury, he's headed to an ancient Incan Empire where he believes he can regain his strength. Dybbuk will stop at nothing . . . even if it means destroying the rain forest, opening a cursed portal, and disturbing the enchanted kingdom of the Incas that has slept for thousands of years.. Find out more about P.B.Kerr here.