Percy Jackson and the Olympians is the first series written by Rick Riordan, and the predecessor of The Heroes of Olympus series. The series focuses on Greek mythology. Every book follows Percy Jackson's adventures in his point of view . Each book is loosely based on a different Greek hero's adventures. The first is based on Perseus' heroics. The second is based on Odysseus' story, The Odyssey. The third, on Hercules', the fourth, on Theseus', and the last, on Achilles'.