In my opinion, each book is better than its preceder, with the exception of Sea of Monsters. Although, this is where it starts to get a little annoying that every stinkin' monster is impossible to defeat, when of course they defeat it anyways.
I was glad that Rick made one of the books about the Labyrinth, and I liked how it was made to be a living maze that always was expanding, and while I'm on it, now is a good time to mention how clever it was to make the Labyrinth, Mt. Olympus, and other Greek mythological places move over to America. Though I wonder, how does this make people in other countries feel when they read it?