Takeaway: Some new friends, some characters die.
The series is aging. Percy is now in 8th grade. Annabeth is trying to live with her Dad and Stepmom again. Tyson has found a home and Grover has found two new children of the Gods that need to make their way to Camp Half-Blood. Percy is still the main and center character, but he is not in charge of the quest, in fact he is chosen to join the quest at all.
The prophesy about the curse says someone will die (and someone does). Luke (the ongoing half-blood bad guy) is back again with new Titan Gods that want to overthrow Olympus.
But when Annabeth goes missing, Percy realizes he cannot sit this quest out and sneaks along, more to save Annabeth than to fulfill the quest.
I really am very impressed with the progression the series. It is just not a routine every book is the same, like I felt that the Lemony Snicket series ended up being. This is a natural, but varied series and it is building quite well. Now on to book 4.