The Titan's Curse by Rick Riordan
When Artemis goes missing on her hunting trip, the kids at camp Half-Blood search for the goddess and the monster that she had set out to fight. They find that Artemis has been tricked by the Titan Atlas and now Atlas is trying to help Kronos. If they don'y free Artemis and stop Atlas fast, they will have less of a chance to win the upcoming battle.
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