InterWorld is a trilogy of fantasy and science fiction novels written by Neil Gaiman, Michael Reaves and later Mallory Reaves. The books are published by EOS and HarperTeen, imprints of HarperCollins Publishers.
Novels
InterWorld
Joey Harker is a very average kid who discovers that his world is only one of an infinite alternate Earths. Some of these Earths were ruled by magic, some by science, but all were at war.
Joey teams up with alternate versions of himself from an array of these worlds. Together, the army of Joeys battle evil magicians Lord Dogknife and Lady Indigo from HEX and 01101 from Binary to keep the balance of power between all the earths stable.
The Silver Dream
In this novel, Joey continues to struggle to find his place within the InterWorld organization, as he is still blamed for the death of a popular team member. As the team begins to lose more and more ground to Hex and Binary, Joey encounters a mysterious young woman named Acacia Jones, whose powers to jump between dimensions seem to rival or even exceed his own and who may not be working for the same ends that he is.
Eternity's Wheel
In his final adventure, Joey Harker is forced to be a leader as he had to step up to face the ultimate enemy who was the mastermind behind HEX and Binary if he has any hope of saving InterWorld, the multiverse, and everything in between.
Setting
Altiverse
The novels are set in the Altiverse, a 'portion' in the multiverse composed of a trillion alternate Earths. Whenever consciousness, not to be mixed with ego, has to make an important decision, the universe branches off in two directions.
The Altiverse is conceptualized as an arc. At one end science reigns, while at the other magic reigns. The reality Joey comes from, which is based on the real world, lies near the center a bit to the science side. The Altiverse has 3 factions fighting an eternal war. Science's region is mostly dominated by Binary ('01101'b). HEX is the magical cabal that controls several of the Magical regions. InterWorld is a guerrilla-like organization that fights to keep the other two groups from dominating the arc, while trying to maintain balance between magic and science.
Besides parallel worlds the Altiverse has bizarre zones named Nowhere-at-all, Static and In-Between. They serve as the inter-plane bridge. The Nowhere-at-all is filled with Magic, while the Static has Science. Traveling from world to world takes weeks or days, except through the In-Between, but only Walkers can go there. The In-Between is also the place where strange multidimensional beings dwell. In any case, these zones aren't really understood.
Walkers
Walkers are special people which possess the gift of "walk" through realities. The only known Walkers are Joseph Harker and all of his alternate counterparts.