It's The lightning thief, The sea of monsters, The titan's curse, The battle of Labyrinth, and Percy Jackson's Greek Gods. (That is the sequence of the book series.)
The book series of Percy Jackson and the Olympians is Percy Jackson and the Ligthing Thief, Percy Jackson and the Sea Of Monsters, Percy Jackson and the Titan's Cures, Percy Jackson and the Labyrinth, and finall, Percy Jackson and the Last Olympian.
