I would say that this is the hardest book to get through in the series simply because of how much stuff (both good and bad) happens.

Bianca and Zoe’s deaths were so shocking to me when I was a tiny 10y.o zooming through this book

Percy’s flaws (if you can call them that) shine in this book, although I would say they were normal decisions and mistakes that a kid would make in his circumstances. You also, however, saw his good qualities at their best. 
Also… Nico D’Angelo. That’s all I can say