I am loving Percy Jackson! Rick Riordan has really done a fine job of blending the old Greek myths into modern times, and his characters are great. Tyson is one of my favorites, he's such a sweet clumsy kid--who knows how many kids like that are really cyclops in disguise!
In this, the second of the Percy Jackson books, Rick Riordan wonderfully captures the horror of some of the classic monsters: Scylla, Charybdis, the Sirens. I don't know how much kids know of the Greek myths these days, but if they didn't know about them before they'll know about them now. So not only are the Percy Jackson books entertaining, but they subtly inject some classical education in there too.
Rick also makes this a great series for boys and girls by introducing some strong female characters too. At the end of the book you may meet the most compelling one yet.