Percy Jackson and the Olympians #5: The Last Olympian by Rick Riordan
All year the half-bloods 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.