Books like The City of Gold and Lead

Having by the skin of their teeth survived a cross-continental walk to freedom from the mind-control of the Tripods, Will and his friends now volunteer to risk their lives by joining those who enter the alien city as slaves of the Masters. First they must make another arduous journey and then triumph at an athletic competition designed to select the best and strongest specimens to serve the invaders. But getting into the city is the least horrible of the dangers that wait for the boys... Tense and creepy.