Ooh, shit got real in this one! So far, I think this is the best book in the Percy Jackson series that I have read.
As always, I will discuss characters first. Percy is likable as always. I'm disappointed there wasn't more of Annabeth. Thalia and Zoe and Bianca were all really excellent characters that I really liked. And Grover is so adorable! Also, we get our first introduction to a few gods, one of whom is Apollo. I read The Hidden Oracle before reading this was, so I already had met Riordan's version of Apollo. And, despite his flaws I love him. I was so happy to see him again! Bottom line? I love the characters of this series.
Also, I really liked the ending. It showed that things were getting serious, a few new mysteries popped up, and that last sentence was kind of perfect. At least in my mind.
Tl;dr version- I can't even talk calmly about this book, I just had so much fun and enjoyed it so much. Now onto the next one!