This is a sequel to Magicians which came out a couple of years ago, and a book I really enjoyed as a dark blend of Harry Potter and Narnia. This was definitely not a book for children and got very mixed reactions. I would recommend you read this before The Magician King.
In Magicians we ended up with our `hero' Quentin, as one of the four ruling kings and queens of the magical land Fillory, but he is a little bored and looking for an adventure. A simple quest to a faraway isle seems ideal, they haven't paid their taxes in a while, but Quentin just wants a little adventure....
With further nods to Narnia this has a lot of Voyage of the Dawn Treader about it, but again with a dark twist. And this is not just Quentin's story, but also fellow queen Julia, the Hedgewitch, and her story becomes more important as it appears that the magical lands may be doomed and Quentin's quest becomes a lot bigger then he thought.
Author Grossman gives you much you are familiar with, especially Narnia fans, but then shakes that familiarity by taking you (and his characters) in some very strange directions. It's great fun and the underlying darkness does not let you get complacent with the direction of the story, Quentin's desire to be a proper hero, Julia's need to find herself and the consequences of their actions.
Sometimes it is not comfortable reading but it was a delight to see how Grossman plays with the readers assumptions and twists what they are used to. There are hints of a third book from the author and I would love to see how he would close this off as a trilogy, all I do know is that he will do it in an original and unique way.