5 books in series

The Trials of Apollo

The Trials of Apollo is a 8-book series with 5 released primary works first released in 2013 with contributions by Rick Riordan.

Percy Jackson and the Singer of Apollo
The Hidden Oracle
The Dark Prophecy
Campo Mezzosangue - Il libro segreto
The Burning Maze
The Tyrant's Tomb
Camp Jupiter Classified: A Probatio's Journal
The Tower of Nero