Enjoyable, slight, fairly good but not deep YA. I do enjoy the ways he weaves Greek mythology into these books, and it's done well, but they don't really seem to add up to anything. I'm not moved or touched by anything that goes on, I feel no sense of urgency - even when characters die, as one did in this book, I didn't really care.
They're enjoyable to read as book popcorn, but don't pack the heft of The Hunger Games, or any Phillip Pullman. If anyone wants a master course in how to kill a character and make it devastating in young adult fiction, read Pullman. (The jerk.)
So, in this one, Percy is on a quest to find his friend Annabeth, along with Thalia, Grover, and two of Artemis' Huntresses. It's a trek across America, complete with a stop at the Hoover Dam.
At the end, the heroes are faced with a choice that is supposed to be difficult, but isn't really, and we have no reason to believe they'd make a different one.
But I do enjoy the Greek mythology. Riordan is very clever about it.