InterWorld

Joey Harker can't tell left from right and gets lost walking to school, so when a field trip sends him into a strange fog, he assumes it's more of the same -- until he emerges into a version of his town where he doesn't exist, replaced by a girl named Josephine. Rescued by a masked stranger named Jay, Joey learns he's a rare 'Walker,' able to move between the parallel Earths of the Altiverse, recruited into InterWorld, an organization holding the balance between magic and science. Training means meeting versions of himself from other realities, staying ahead of enemies hunting them. A novel by Neil Gaiman and Michael Reaves.