"The Never War" by D. J. MacHale (Pendragon, Book 3)

The third installment in an epic series of adventures

First Earth
Fourteen-year-old Bobby Pendragon is a loyal friend, sports star, devoted pet owner -- and Traveler. Along with his uncle Press, Bobby has visited the alternate dimension of Denduron and participated in a civil war. He's also waded through the endangered underwater territory of Cloral. Now Bobby once again finds himself thrust beyond the boundaries of time and space into a place that seems somewhat familiar: First Earth.
Bobby and the Traveler from Cloral -- Spader -- have flumed to New York City, 1937. Against a backdrop of gangsters, swing music, and the distant sound of a brewing war, the two must uncover the evil Saint Dane's newest plot. But is Bobby ready for the difficult choices ahead?