There are five books in the Percy Jackson & the Olympians series and are as follows:

1. The Lightning Thief is the first book in the series and was released on July 1, 2005.

2. The Sea of Monsters is the second installment in the series, released on April 1, 2006.

3. The Titan's Curse is the third installment in the series. It was released on May 11, 2007.

4. The Battle of the Labyrinth is the fourth installment in the series. It was released on May 6, 2008.

5. The Last Olympian, the fifth and final book in the Percy Jackson series, was released on May 5, 2009.

More Info: en.wikipedia.org