Split Infinity starts out the Apprentice Adept series with the introduction of Stile, a serf on the world of Proton. Someone is trying to kill him, and he doesn't understand why. Then he stumbles across the magical frame of Phase, where someone is still trying to kill him. Some people have all the luck.

Stile finds that he can use incredible magic when he is in Phase. However, none of his friends want him to do this, as it would attract the attention of Adepts, or he might become one, and in their experience all Adepts are hostile. And when a unicorn and a werewolf, your only friends, don't want you to use magic, that's something you really have to be careful about.

Stile has to go between the two worlds because he is competing to be allowed to remain on Proton as a Citizen rather than a serf. Otherwise he will have to leave forever. 

As with most of Anthony's work, younger readers are more likely to enjoy these stories than older readers. Call it high school age or so, since there's a fair amount of mild sex in the book.