Magician King - Lev Grossman

Quentin and his friends are now the kings and queens of the magical land of Fillory. But he finds himself growing restless. So when a mystery at the heart of the kingdom is uncovered, Quentin and his old friend Julia charter a sailing ship and set off to the wild outer reaches.But their voyage will take them much further than they imagined and leave them with a choice that could have devastating consequences - for Fillory and for magic itself.