The lost city of Faar by D.J. MacHale

Fourteen-year-old Bobby Pendragon is not like other boys his age. His uncle Press is a Traveler, responsible for solving interdimensional conflict wherever he encounters it -- and he's taking Bobby along for the ride. Fresh from his first adventure on Denduron, Bobby finds himself in the territory of Cloral, whose desperate citizens are on the brink of war. Can Bobby help rid the area of marauders, and locate the legendary lost land of Faar, which may hold the key to Cloral's survival?