It seems like the even-numbered books in this series are written with more thought, attention to detail, and interest in the characters. 
 I thoroughly enjoyed this one. I loved everyone (except for the Calypso storyline but whatever). All the characters I didn't like before, I enjoy now. Mr Riordan took time to flesh out the characters, color them in, make me invested in their quest and personal histories. 
 Jason. Frank. Hazel. Piper: I looked forward to their chapters and enjoyed their POV. 
 It goes without saying that Percy and Annabeth were my favorites. Looooooooved their chapters. And I love that their character grew by being forced to see things from monsters POV. I loved that they had to examine themselves about what they considered monsters. 
 Leo..... I liked Leo, but as stated above, his whole storyline with Calypso is meh to me (and kinda icky), so yeah. 
 Nico is an interesting character. I am glad Mr Riordan included him as gay character, but I don't know what to make of his unrequited love storyline. It is one thing to be gay and like someone but be afraid to share your feelings because of a homophobic society. There is also the issue of reciprocity. It's a whole nother thing to be gay and have unrequited love for someone else who is clearly only heterosexual. IMO that doesn't count as unrequited love because there is no possibility of them getting together......I dunno. Then there is the issue of reciprocity. One cannot force love, so while it's sad that ones love is unreturned, it is what it is. Unreturned love is very different from forbidden love. However it felt like the book was going for a "forbidden love" vibe, which is simply untrue. So while I am glad Mr Riordan breaks from heteronormativity, I am not too sure about the direction Nico's story has taken. 
 But all in all a STELLAR book! Far cry from books 1 and 3, and almost as excellent as book 2.