I really like the added layers that Riordan puts into each new installment of this series. It gives this world a sense of depth and realism, and it's clear he planned this out ahead of time. It also gives it a sense of mystery, such as the mortals who could see through the Mist and who exactly took Bianca and Nico to Westover Hall. I'm also enjoying the slow build of tension between the Olympian gods and the Titans. Seeing more of the gods getting involved and not being completely aloof and hands off is good too, as I was getting frustrated in previous books about their no-interference rule.
Thalia is self-assured and strong, and I liked that about her. Zoe is as well, and even though the two don't get along, I appreciated they didn't turn into squabbling girls throughout the book. They bickered now and again, but they were able to put aside their differences and work together on the quest. Thalia's conflict with her father was given room here and it would've been easy to understand if she'd made a different choice than she did. I really liked that everyone had their moment to shine and it wasn't just Percy doing everything.
The middle seemed to lag a little and that whole sequence with Bianca in the junkyard seemed a little too contrived in order to give us the surprise reveal at the end. The zombies were sufficiently frightening and Blackjack was a hoot. Bessie was a sweetie but hopefully won't be seen again.