The Magician King By Lev Grossman

Quentin Coldwater should be happy. He escaped a miserable Brooklyn childhood, matriculated at a secret college for magic, and graduated to discover that Fillory—a fictional utopia—was actually real. But even as a Fillorian king, Quentin finds little peace. His old restlessness returns, and he longs for the thrills a heroic quest can bring.

Accompanied by his oldest friend, Julia, Quentin sets off—only to somehow wind up back in the real world and not in Fillory, as they’d hoped. As they struggle to find their way back to their lost kingdom, they rely on Julia’s illicitly learned sorcery to fight a sinister threat in a world unlike the fantasy novels of their youth.