Percy Jackson and the Battle of the Labyrinth by Rick Riordan: The fourth installment of the Percy Jackson and the Olympians series is action-packed, funny, clever, and original, with character and plot developments we've been waiting for since Book 1, The Lightning Thief. Percy is a clever boy (I think he's 12 in book 1) who's been kicked out of multiple schools, has ADHD and dyslexia, and a knack for trouble. In The Lightning Thief, he learns he's a half-blood, the son of a Greek god, and discovers there are others like him at Camp Half Blood. If you haven't read the series, I don't want to give away too many details of the plot of Book 4. Battle of the Labyrinth opens with Percy's orientation at his new school, which is equipped with monster cheerleaders (yes, that's literal) before heading off to Camp Half-Blood, which is in danger like never before. A chunk of this installment takes place in the Labyrinth, the monster-infested, ever-changing maze underneath the U.S. Any juvenile fantasy is compared to Harry Potter these days, and I have to say, I love both series. Percy Jackson is not derivative of Harry Potter, and Riordan manages a fresh and funny take on ancient stories (the Greek myths). I can't recommend this series and this installment highly enough, and I can't wait for Book 5 (and it's very clear there will be a Book 5, though I believe that will be the last).
Edited to add: I went looking for the source of my belief that there will be five books (sadly it's true--next year's will be the last, though the first film will be out next year, too), and discovered a short story that's available on Rick Riordan's blog. Here are the three parts (part 1 is at the bottom of the page). Here's fun bonus Percy!