Another Camp Half-Blood series has been released, titled The Heroes of Olympus. The Lost Hero is the first book in this series by Riordan, and was released on October 12, 2010. Though it is not directly related to Percy and friends, there are many references and appearances of these characters and it still serves as a sequel. Its sequel, The Son of Neptune, portrays Percy as one of the main protagonists. The Son of Neptune is followed by The Mark of Athena, The House of Hades, and The Blood of Olympus.