I think this book is the best for the side characters. Annabeth and Grover in particular really get to shine. Annabeth finally gets her own quest in the labyrinth while Grover is continuing his search for Pan. Other than The Lightning Thief, this book might have the most worldbuilding in it. The labyrinth is a great tool for seeing new pockets of mythology in the US that have previously gone unnoticed. Also, this is one of the most comedic books in that Percy interacted with no less than four people who had crushes on him and had no idea. Going back to the side characters, Nico and Rachel got some amazing development, and while you'll know something's up with Quintus, I doubt you'll be able to guess what it is (I actually totally forgot about it so I got to be surprised all over again). Also, Mrs. O'Leary my beloved.