The Merchant of Death: The Pendragon Series, Book 1

When Bobby Pendragon is taken by his Uncle Press to an abandoned subway station in the Bronx, he discovers that he, like his uncle, is a Traveler, with the ability to ride "flumes" through space and time. Press takes Bobby through a flume to a world called Denduron, where an evil Traveler called Saint Dane is inciting war between the ruling Bedoowan and the virtually enslaved Milago miners. But almost as soon as they arrive, Press is captured by the Bedoowan and scheduled for execution.

Bobby doesn't understand anything of what's going on, and all he wants to do is go home. But, challenged by Loor, a warrior-Traveler from another world, he hatches a plan to rescue Press from the Bedoowan stronghold. Getting Press back, though, is only the beginning of stopping Saint Dane's plans for sowing chaos across the planet and throughout the universe.