The Heroes of Olympus: The Demigod Diaries

The Demigod Diaries - Rick Riordan A filler, an in-between book, that provides some addition background to the characters lives / stories from the 'Percy Jackson' series, and the 'Heroes Of Olympus' series -- but not really adding anything to the main plot, although some of the information might be used in later installations of the 'Heroes Of Olympus' series.