The first of the Rick Riordan Camp Half-Blood series. This series has five books, The Lightning Thief, The Sea of Monsters, The Titan's Curse, The Battle of the Labyrinth, and The Last Olympian with companion books like The Demigod Files, The Ultimate Guide, and Demigods and Monsters. Every book follows Percy Jackson's adventures. Each book is loosely based on a different Greek hero's adventures. The first is based on Perseus' heroics. The second is based on Odyseus's story, the Odyssey. The third, on Hercules, The fourth, on Theseus. And the last, on Paris and Achilles.
In the acknowledgments section of The Last Olympian, revealed that Percy Jackson and the Olympians is only the first of the Camp Half-Blood series.
The next series is The Heroes of Olympus which starts with The Lost Hero, whick was released on October 12th, 2010, followed by The Son of Neptune, which was released on October 4th, 2011. The third book, The Mark of Athena, is set to be released in Fall of 2012.