The Battle of the Labyrinth (Percy Jackson and the Olympians, Book 4) - Rick Riordan
Thoroughly enjoyed this book for a number of reasons.
I think this is the installment where everyone grew up. They let people they never thought they would lose, like Annabeth lost Luke. The heroes had to inspire their own heroes to do the right thing. I felt so bad for Tyson, thinking Briares was going to let him down. And I'm glad Annabeth got Daedalus's laptop. If anyone deserved it, she did.
Nico relief he had to let go of his sister. He handled it well, I think.
Grover finally found Pan, just to lose him, which is a heartbreaking thought if only because it's so true that we're losing all the wild places.
The heroes are developing personally. I never thought Clarisse would care enough for anyone to take care of them the way she did. The heroes are relying less and less on Chiron. They're making their own decisions.
Looking forward to the final book.