The PJO series is an adventure/fantasy series authored by Rick Riordan. It's set in the U.S and is based on the Greek Myths. (The Gods, Creatures, Titans... etc..)
The main character is Percy Jackson. He discovers that he is the son of Poseidon, the God of the Sea and Earthquakes. He finds out that all the myths and Greek Mythology actually does exist. (Monsters, Titans, Olympians, etc,) The Olympians reside in Olympus, which is now stationed on the 600th floor of the Empire State Building. Throughout the book series his two best friends Grover (a satyr) and Annabeth (A child of Athena) accompany him on many quests and adventures.
The children of the Gods are called Demi-Gods: Half-human, half God. They stay at a Camp called Camp
Half-Blood where they learn to fight and defend themselves. They learn more about their Olympian
Parents and their powers.

The Percy Jackson series consist of five books by Rick Riordan:
Percy Jackson and the Lightning Thief
Percy Jackson and the Sea of Monsters
Percy Jackson and the Titan's Curse
Percy Jackson and the Battle of the Labyrinth
Percy Jackson and the Last Olympian