The second novel in the Percy Jackson & The Olympians series is, like the first, very very good.
This time, the thirteen year-old Percy Jackson is accompanied by Annabeth (again) and Tyson - a very tall and ugly and socially rejected kid that attended school with Percy this year - and once again a quest! appears. Yes, it involves Grover. This new quest more or less follows Odysseus' odyssey, and by the end of the book the plot surrounding the eeeeevil titan Kronos thickens.
Now, I have to draw the inevitable parallels between Percy Jackson's series and Harry Potter's: in both cases we have a heroic but not perfect hero (Percy), who has an extremely smart female friend (Annabeth), a bit of a comic relief of a best friend (Grover), whose adventures take place at a specific time of year (during the summer), feature a magical location away from his parent(s) (Half-Blood camp), and... It doesn't matter. Because these resemblances in no way detract from the story's enjoyability.