Interworld

A 2007 Science Fiction novel by Neil Gaiman and Michael Reaves about Joey, a boy who finds out the hard way that he is a "Walker", meaning he can jump between the many alternate universes. The good news: no Evil Twin. The bad news: not one, but two Evil Empires trying to take over the multiverse. Long story short, Joey ends up joining an inter-universal army whose mission is to keep neutral realities safe in the war of Magic Versus Science.

Tropes used in Interworld include:

Acid Trip Dimension: The world inbetween universes is described as this.
All the Myriad Ways: Every choice with results creates a new reality where the opposite action was taken. Minor events do not spawn new realities, or if they do, the new reality fades away rapidly.
Alternate Universe: Hundreds of thousands of them, from slight shift of timeline (where Joey dies from a dare) to the evolution of human beings being different.
And I Must Scream: The Walkers used to fuel HEX ships are still alive and conscious, despite being reduced to almost nothing.
And the Adventure Continues...: The book ends with Joey and his team becoming official members of InterWorld and leaving on their next assignment.
Because You Were Nice to Me: Hue helps out Joey because Joey saved its life in the beginning.
Big Bad: DogKnife, leader of Hex army.
Butterfly of Doom: Only major changes cause new realities to form, but major changes can be caused by seemingly minor actions. As described in the book, if a butterfly can eventually cause a hurricane, then stepping on the butterfuly before it can flap its wings might be enough to split reality.
Chekhov's Skill: The winged Joey can use her wings to slow herself and someone she is carrying down while falling, even if she's on a world where she can't fly.
The Chosen One: It turns out that all of the Walkers are the Joey's from different realities, even the ones that are older or female.
Cyborg: One of the Walkers comes from a world where everyone is cyberized at birth.
Depth Deception: Due to existing beyond the usual 4 dimensions, nobody can be certain just how large MDLFs are. For example, Hue is mentioned to either be the size of a housecat from 2 feet away, to the size of a skyscraper from several miles away, to the size of the state of Vermont from several thousand miles away.
Eldritch Abomination: The multidimensional life forms (aka MDLFs or mudluffs).
Evil Versus Evil: Neither Hex nor Binary remotely resemble good guys.
The Fair Folk: Fairy tales are based on beings from Hex.
Fantasy Kitchen Sink: Thanks to creatures from Hex popping into Joey's Earth on occasion.
Five-Man Band: Joey's team of six is kinda this.
- The Hero: Joey himself, due to being the protagonist as well as having the "greatest potential as a Walker".
- The Lancer: Varies. Usually, it's Jakon, Jai, or Jo, as they all seem to be capable of being "second in command". May also be J/O, due to his kinda snarky nature.
- The Smart Guy: Both Jai (Sesquipedalian Loquaciousness as well as psychic powers) and J/O, due to having an immense amount of knowledge (such as advanced swordfighting) uploaded into his brain.
- The Big Guy: Josef. Coming from an Earth with more than twice the gravity of "our" Earth, he's quite large and muscular.
- The Chick: Sometimes Joey, due to the rest of his teammates having some special ability.
- Team Pet: Hue the mudluff. He (or she, it's hard to tell with multi-dimensional beings) even has a habit of floating a foot or two above Joey's shoulder.
Fluffy Tamer: The first thing Joey does in the In-Between is make friends with a MDLF, which he names Hue.
The Greys: The stories are caused by the Binary making incursions into the world.
Hyperspace: Used to travel between realities and where InterWorld is located.
Instant Awesome, Just Add Dragons: At one point, Scarabus summons a dragon to attack the team.
Laser-Guided Amnesia: InterWorld is able to selectively wipe the memories of people who leave.
Magic Versus Science. The plot of the book. Basically the empire of science (the Binaries) is at war with the empire of magic (Hex) and both are trying to take over all realities.
The Magic Versus Technology War: A war that has been going on for a long time. However, most realities are non-aligned regardless of where they fall on the spectrum.
Mark of the Beast: The Mook Scarabus is covered in tattoos, which he presses to summon whatever the tattoo depicts.
Me's a Crowd: Well, they're all from an Alternate Universe, but they're all the same person.
Meaningful Name: Hue is named so because he communicates via shifting colors.
Mind Wipe: InterWorld cadets who wash out have their memories erased before being sent home.
Mr. Exposition: The mooks like to describe their plans in detail to characters who should already be familiar with what's going on.
Nightmare Fuel: The Mook Neville has transparent skin, in addition to being an unpleasant person.
No Sense of Direction: Joey has gotten lost in his own house. Turns out that it's because his brain isn't wired for three-dimensional travel.
Not So Different: Hex and Binary have the same goals, abilities, and treat humans like animals. For example, one Hex wants to boil down the Walkers' bodies until only their soul are left; the other wants to slowly drain out his energy over the same period while keeping him in cold storage.
Painting the Fourth Wall: When Joey has his memories erased, one of the pages goes completely black.
Powered by a Forsaken Child: The engined used by Hex and Binary are fueled by the souls of captured Walkers.
Pragmatic Adaptation: The sequence to enter InterWorld is represented as a mathematical formula in the book, but is changed to a musical chime in the audiobook.
Roaring Rampage of Revenge: When the imprisoned Walkers are set free, they destroy the ship in a matter of minutes.
Rubber Forehead Aliens: Some of the alternates have features of other animals, such as wings or fangs. Justified in that they come from the more magical Earths, where magic might have been used.
Science Fantasy: Intentionally in the grey area between fantasy and soft Science Fiction
Sequel Hook: Neither Hex nor Binary are defeated, and mention of a mysterious HEX operation called FrostNight is made, but no sequel was ever written.
Sergeant Rock: The head of the InterWorld military. Gruff and cold, but cares deeply about his troops.
Sesquipedalian Loquaciousness: One of Joey's teammates speaks this way because "what's the point of having words if you're not going to use them". Even the cyborg with several implanted dictionaries has trouble keeping up at times.
The Sliding Scale of Magic Versus Technology: All Earths fall somewhere on the spectrum (refereed to as "the arc"), with Joey's Earth being near the middle, but slightly on the science side.
Space Police: Well, inter-reality police, but functions in much the same way.
The Spartan Way: How InterWorld members are trained.
Time Dilation: Time does not pass the same way on all realities, and results in both Rip Van Winkle and Year Outside, Hour Inside. The InterWorld organization has some control over the effects when leaving the In-Between.
Took a Level in Badass: Joey, after months of training. In addition to being physically strong, he's been taught such advanced science and math that he goes from being a remedial student to one of the best. Since this is after the Mind Wipe, he has no idea why he can suddenly do so well.
Winged Humanoid: One of Joey's teammates has angel-like wings. Since they function by magic, they tend not to work well on the more science-y worlds.
Year Outside, Hour Inside: When Joey returns to his world after months of training and having his memory erased, only about 36 hours have passed.
You Can't Go Home Again: Joey can never return to his home dimension, lest HEX or The Binary track him there and attempt to take over, possibly killing his family in the process.