Un Lun Dun, by China Mieville
Mieville is my favorite living author, so I’m a bit biased, but this is such an amazing and revolutionary take on youth fantasy novels. It’s about the Chosen Girl, Zanna, and her sidekick Deeba who go to the city underneath London (UnLondon) to save it from the evil smog villain. Then the Chosen Girl gets hit on the head and has to go home, the Book of Prophecy has an existential crisis, and Deeba is taught all the rules of fantastical adventures and saves the day by ignoring them. Also the wordplay is just amazing. For example, broken umbrellas make their way into UnLondon and are then unbrellas, under control of the Generalissimo Brokkenbroll. If you fix one, it becomes an independent agent and is then a rebrella. SO COOL.