Still not disappointed! That's remarkable for a middle-of-the-series book too. Riordan manages that Rowling balance of keeping each story feeling somewhat contained to the bounds of that volume, while hinting at much larger goings-on, usually in the last chapter. The selling point here is the character development. I don't know when it happened exactly, but I'm caring a lot about what happens to these kids. I'm also liking how Riordan handles the whole prophecy thing. He introduced it as a way to add complexity and complex foreshadowing and he continues to fulfill the prophecies in, I wouldn't say startling, but certainly inventive ways that put him above "amateur." Prophecy and determinism take center-stage in this one and the plot definitely thickens significantly. I think this is a really smart series for really smart kids.
The only thing I'm having trouble buying is just how young these guys are. I don't think I've ever thought of them as 10 or 12 years old since the series began. Their dialogue, feelings and the internal musings of Percy himself seem far too sophisticated for someone that age group.