Percy Jackson and the Battle of the Labyrinth
Rick Riordan
Hyperion

The last thing Percy wants to do on his summer holiday is blow up another school. But after doing exactly that, he is forced to flee to Camp Half Blood, where he finds that all is not well. His arch-enemy Luke is looking for a way to invade the camp via an ancient labyrinth and if he succeeds, thousands of bloodthirsty monsters will attack. So it is goodbye sunshine, hello darkness as Percy and his friends descend into the terrifying underground...

Also in the same series:
Percy Jackson and the Lightning Thief
Percy Jackson and the Sea of Monsters
Percy Jackson and the Titan’s Curse
Percy Jackson and the Last Olympian
Percy Jackson: The Demigod Files