The Titan's Curse (Percy Jackson & Olympians 3): Rick Riordan

When Percy Jackson receives a distress call from his friend Grover, he immediately prepares for battle. He knows he'll need his powerful demigod allies, Annabeth and Thalia, at his side; his trusty broze sword Riptide; and... a ride from his mom. The demigods race to the rescue, to find that Grover has made an important discovery: two new powerful half-bloods whose parentage is unknown. But that's not all that awaits them. The Titan lord, Kronos, has set up his most devious trap yet, and the young heroes have unwittingly fallen prey. Hilarious and action-packed, this third adventure in the series finds Percy faced with his most dangerous challenge so far: the chilling prophecy of the Titan's curse.