Pretty much they are reading all the books by Rick Riordan. This shouldn't be necessary, but SPOILERS to Trials of Apollo. First Book (this one): Percy Jackson and the Olympians Series Second Book: Heroes of Olympus Series Third Book: Trials of Apollo Series (This one won't be marked as complete until all the books in the series are out) Fourth Book: Kane Chronicles Series Fifth Book: Magnus Chase and the Gods of Asgard Series Sixth Book: All the extra books. (This will probably never be marked as complete as there are extra books constantly coming out) DISCLAIMER: I DO NOT OWN PERCY JACKSON AND THE LIGHTNING THIEF. RICK DOES!All Rights Reserved