Percy Jackson and the Olympians #4 The Battle of the Labyrinth by Rick Riordan*

When reading this book to himself, my twelve year old actually limited how many pages he could read each day because he does not want this series to end!!
This book focuses on the battle between the Olympians and the Titans.  Percy and friends also go on a quest in the Labyrinth to prevent the Titans from invading Camp Halfblood.
*Highly recommend by my twelve year old.
Warning/points to discuss: worshiping gods, underworld