The Walls of the Universe — Paul Melko

John Rayburn thought all of his problems were the mundane ones of an Ohio farm boy in his last year in high school. Then his doppelgänger appeared, tempted him with a device that let him travel across worlds, and stole his life from him. John soon finds himself caroming through universes, unable to return home--the device is broken. John settles in a new universe to unravel its secrets and fix it.

Meanwhile, his doppelgänger tries to exploit the commercial technology he's stolen from other Earths: the Rubik's Cube! John's attempts to lie low in his new universe backfire when he inadvertently introduces pinball. It becomes a huge success. Both actions draw the notice of other, more dangerous travelers, who are exploiting worlds for ominous purposes. Fast-paced and exciting, this is SF adventure at its best from a rising star.