Percy Jackson and the Olympians, Book Five: The Last Olympian By Rick Riordan

All year the demigods have been preparing for battle against the Titans, knowing the odds are against them. Kronos is stronger than ever, and with every god and half-blood he recruits, his power only grows.

In this momentous final book in the 
New York Times best-selling series, the prophecy surrounding Percy’s sixteenth birthday unfolds. And as the battle for Western civilization rages on the streets of Manhattan, Percy faces a terrifying suspicion that he may be fighting against his own fate.