Warning: Again, beware spoilers if you haven't read the previous three books.
In the spectrum of the series, this fourth installment (in my opinion) isn't as fun as the third, but it's still pretty good. As the previous book took place during the winter, this one picks up the following summer, going back to the formula that the first two books used (and the third mostly skipped over). Percy is about to start high school, and his mom's new boyfriend pulls some strings to get him enrolled in his school. Though during orientation, Percy stumbles into another new student, somebody he met the previous winter--Rachel Elizabeth Dare--a mortal girl with the ability to see through the Mist. Needless to say, havoc ensues, and Percy is forced to flee with a visiting Annabeth, and they make their way to camp.
Once at camp, they see Dionysus is busy doing godly things, and a swordsman named Quintus is there to help out Chiron for the summer. Grover is in trouble with his elders, as he can't back up having sensed Pan last winter. Nico di Angelo is still missing, and Percy keeps receiving disturbing Iris-messages that show what Nico is up to. But then Percy stumbles upon a secret entrance within the camp boundaries... an entrance to the massive, underground Labyrinth. And this brings up many new issues--they realize that Luke can bring forth his armies into camp this way, getting around its defenses. But they know it can also help find Pan somehow. So now a new quest begins, led this time by Annabeth, into the Labyrinth to find its creator--Daedalus--before Luke does, to persuade him not to help Luke find his way through the Labyrinth.
If you've noticed, there are really two plots to this one, the "stop Luke" plot and the "Pan" plot, though the Pan plot is more secondary here. Important, but secondary. To cut right to the negative, what I didn't like the most about the book was how it was a bit all over the place. Like the Labyrinth itself, the book was, at times, chaotic. But I don't fault it for that. My issue lies in how that chaos comes off as too organized. Half the book feels forced, and the other half feels rushed. It's like there were a bunch of ideas the author wanted to put into the book, but (with most of them) he wouldn't give them the amount of time they deserved. And then, as I said, some of the things just felt forced in there, just there to go "hey, I'm important to the plot later, but I just needed to be introduced now so it makes sense when the time comes. kthxbai." I don't know. I just think there was a good chunk of the book that was handled too sloppily.
That being said, the book has a strong first few chapters and a strong last half (or thereabouts). There are a couple really good twists toward the end. And I knew way ahead of time that these characters would be involved in twists, I just didn't know how. And no, I'm not talking about Rachel Elizabeth Dare, though I did love her involvement. Right about the time I started going "okay, what the heck was her point?," the book brings her back into the equation. Though the lead-up to the reveal brought back bad flashbacks to the first book, where Riordan took an unbelievably long time to reveal the completely obvious.
On a related note, I did really enjoy how the book started to focus more on the romantic relationships between some of the characters. Though Percy is completely daft about it all. But still, I can't wait to see it all resolved in the last book.
One thing I loved about the second and third books were the involvement of some really fun gods and goddesses. Unfortunately, the two central ones this time around aren't nearly as interesting as the others have been. Hephaestus is interesting, but his personality is dull. And Hera is barely in the book for me to really care. I wanted to see Hermes or Apollo again. Though there is a great, though much too short, section with Calypso (not a goddess, but immortal) that is utterly heartbreaking. I hope she comes back into the story in the last book, because I really liked her character, and you can't help but feel horribly for her.
Also taking a larger role is Nico di Angelo. The character is annoyingly stubborn and dangerously naive, but I still can't help but like him for how cool his powers and whatnot are. I think the ending of the book bodes well for the last book. Hopefully Nico is a little less annoying and a lot cooler, because he is a character with a lot of potential.
I know I said some negative things here, but I did enjoy the book. In my current list from favorite on down, it'd probably come in second (after Titan's Curse). I should start reading the fifth soon. But as for this one, maybe my expectations were too high after the third. Who knows? It's still a good read.