Battle of the Labyrinth (08) by Riordan, Rick [Paperback (2009)]

As an incoming freshman, Percy isn't expecting his high school orientation to be any fun. But when a mysterious mortal acquaintance appears, followed by demon cheerleaders, things quickly move from bad to worse. In this fourth installment of the blockbuster series, time is running out as war between the Olympians and the evil Titan lord Kronos draws near. Even the safe haven of Camp Half Blood grows more vulnerable by the minute as Kronos's army prepares to invade its once impenetrable borders. To stop them, Percy and his demigod friends will set out on a quest through the Labyrinth-a sprawling underground world with stunning surprises at every turn. Full of humor and heart-pounding action, this fourth book promises to be their most thrilling adventure yet.