Actually, I think it's more like Harry Potter meets Narnia meets The Phantom Tollbooth, for grownups. I don't even want to tell you anything about the plot, because it's so amazing, you'll want that "reading for the first time" feeling to be completely unspoiled. But to start, you have a young man, Quentin, who is about to graduate high school, and feels there is something missing, at some deep, fundamental level, from his life. When he is whisked away to take a test for a very special academy, Brakebills, his life changes in more ways than he could have imagined.
Somehow these books manage to cram in allusions and references, and never once be derivative. Lev Grossman's writing is biting, bitter but funny, and completely captivating.
Even better--in a rare trick for a sequel--is The Magician King, which continues the story of Quentin with his desire to go on a quest. This is to The Voyage of the Dawn Treader what the first book is to The Lion, The Witch, and the Wardrobe--again, very much the grown-up version. Interspersed with that quest is the harrowing story of Julie, Quentin's childhood friend who missed out on the magic of Brakebills college, but found it again on her own, darker path.
I cannot rave about these books enough. Please read them. They are darkly beautiful, captivating, frightening--everything Narnia for grown-ups should be.