Riordan introduces new characters in this first book of a series and it's nice how they will eventually join Percy in future books. For now, Percy is missing and Annabeth, Thalia, and all the half-bloods are looking for him and trying to keep the giants in the ground. Conveniently, three new demigods appear--Piper, daughter of Aphrodite, Jason, son of Jupiter, and Leo, son of Hephaestus. And, yes, I said Jupiter. The twist in this series is that Jason appears--the gods have been hiding their Roman counterparts. Maybe. That will come out in Book Two!
Truthfully, Riordan's editor should have had the guts to tell him to tighten it up--it's about 100 pages too long. My attention wandered and the cross-U.S. quest was so similar to previous books that I grew bored. Piper is the character to watch--Jason and Leo seem to be repeats. But, sure, I'll probably still read Book Two. Eventually. It took me months to get around to reading Book One! :)