InterWorld by Neil Gaiman and Michael Reaves
InterWorld tells the story of Joey Harker, a very average kid who discovers that his world is only one of a trillion alternate earths. Some of these earths are ruled by magic. Some are ruled by science. All are at war.
Joey teams up with alternate versions of himself from an array of these worlds. Together, the army of Joeys must battle evil magicians Lord Dogknife and Lady Indigo to keep the balance of power between all the earths stable. Teens—and tweens and adults—who obsessively read the His Dark Materials and Harry Potter series will be riveted by InterWorld and its sequel, The Silver Dream.
Neil Gaiman is the New York Times bestselling author of the novels Neverwhere, Stardust, American Gods, Coraline, Anansi Boys, The Graveyard Book, Good Omens (with Terry Pratchett), The Ocean at the End of the Lane, and The Truth Is a Cave in the Black Mountains; the Sandman series of graphic novels; and the story collections Smoke and Mirrors, Fragile Things, and Trigger Warning.