The Sea of Monsters, by Rick Riordan, 2006

Percy Jackson almost makes it through a year of school without incident, until fire-hurling Laistrygonian monsters show up in gym class on his last day. His not-so-little little brother, Tyson, saves him, and together they flee with Annabeth to Camp Half Blood, which has had a leadership shakeup and fallen on hard times. Could this be another step in Kronos' bid to return to power, aided by disgruntled heartthrob Half Blood and Percy's old nemesis, Luke? Like good Greek tragedy, a full cast of monsters for our hero to battle; like good middle-grade fantasy, a loyal if complicated band of friends to join in the quest. Fun, fast, furious. Book Two of Percy Jackson and the Olympians is strongly recommended. Read the books in order.