The Mark of Athena is about how Annabeth, with her friends, finds the Athena Parthenon. In the beginning, Annabeth and Percy are reunited and Jason is back home. While they are at the Roman demigod camp, Leo is possessed by an eidola who makes him blow up a building. The Romans get mad and chase the Argo II throughout the book. I really like this book because like in all the books you connect with the characters. And you really must have the next book ready because this one ends on a giant cliffhanger.
Bottom line: They get scarier, because the characters go into creepier places, as they progress. I really like them. They are still amazing. This one is for 5th grade and older.