Percy Jackson and the Olympians: The Titans Curse by Rick Riordan

This is the third book in the Percy Jackson series. It picks up with them going to a school to recruit 2 new demigods for the camp. Then, when Percy's best friend Annabeth gets captured by a monster, Percy and his other friends must go on a quest to save her. At the same time, the Titan force is still rising, and they seek to destroy Camp Halfblood and Olympus. With the lord kronos still growing piece by piece, gods and demigods all around the world are starting to panic. --Madelaine