Firstly, Rick Riordan is just a fantastic author. His stories are always great. As a fan of the Percy Jackson series I started the Heroes of Olympus series simply because it takes place afterwards. The Mark of Athena is the 3rd book in the Heroes of Olympus Series. I can honestly say that this is one of my favorite series, and is definitely as good, if not better than the original Percy Jackson Series. This is a must get for Percy Jackson fans or anyone that enjoys Greek or Roman mythology.
My only complaint with the book is the audience it is written for. I wish Rick Riordan would make his novels less child-ish and more towards young-adults. This is not a deal breaker though, they are appropriate for young readers and are still very enjoyable for young adults or even adults as well.