I just finished BATTLE FOR THE LABYRINTH, the fourth book in RICK RIORDAN's PERCY JACKSON AND THE OLYMPIANS SERIES. My son is wrapping up the fifth and final book right now, and I'm looking forward to wrapping it up too, though I can't quite imagine how Riordan plans to pay off all the developing plot lines in one book.
In Battle for the Labyrinth Luke and his band of evil monsters continue their quest to raise Kronos. This time they intend to also exploit the labyrinth, a continent-spanning underground maze, in order to attack Camp Halfblood.
It sounds silly, but it was quite enjoyable. One of Riordan's conceits for the series is that the major locations in the Greek myths migrate with the center of Western Civilization. (And in typical U.S.-centric thinking, that means they are here in the states.) Mount Olympus, for instance, is reached by going to the top of the Empire State Building. In this book we discover that the labyrinth built by Daedalus underlies and has entrances all over the U.S.