The Heroes of Olympus is a pentalogy of fantasy novels written by American author Rick Riordan. It is the second of four book series in the Camp Half-Blood Chronicles, succeeding Percy Jackson & the Olympians, and preceding The Trials of Apollo and The Nico di Angelo Adventures. The novels detail a conflict between Greek demigods, Roman demigods, and Gaea. The series can be read as a standalone series, but is meant to be read after Percy Jackson & the Olympians. Riordan introduces Roman mythology in the series alongside several new characters, primarily from the Roman Camp Jupiter. The first book of the series, The Lost Hero, was published on October 12, 2010. The final entry in the series, The Blood of Olympus, was published on October 7, 2014. Two sequel series, The Trials of Apollo and The Nico di Angelo Adventures, follow. Source: Wikipedia (en)