This may be my favorite book so far in the Percy Jackson series(ses). It goes back and brings in some of the villains and situations from earlier books, and particularly in the case of Leo and Frank, really brings some change and maturity to the characters. Even Percy himself sees that he handled a situation in an earlier adventure in a way that he's not proud of now. There's a secret about Nico that is quite shocking... but if I keep going I'm going to reveal something you should read the book to find out, like (view spoiler)[[just kidding... no spoiler here!] (hide spoiler)] ...anyway. I was very impressed with this installment. And after what Rick did to us at the end of The Mark of Athena, it's a good thing he delivered on this one! Or I would never have forgiven him! And that would have totally ruined his whole day! Because me! 
 Great book. Read it. Read the others first, though.