This is the second novel in the Percy Jackson and the Olympians series, and begins after Percy receives a message from Grover, a satyr, asking him for help though a dream. Having read a few books in the series now, I've noticed that each one ties in a bit with an ancient hero's quest. For example, this novel contains many references to monsters and creatures that Odysseus faced. That isn't to say that the novels parallel those ancient quests as much as that they draw elements from them. In this case, Percy and his friends are actually on a quest to retrieve the Golden Fleece famous due to Jason (the tree that protects the camp has been poisoned, and the Fleece has healing powers), and since it has ended up on Polyphemeus's island, the Cyclops that Odysseus defeated, it makes sense that Percy and his friends would run into several of the same monsters as Odysseus - they live in the same neighorhood after all. Unfortunately, my memory of Greek mythology is shaky enough so that while I understood the parallels in this and the next two novels, I'm still not sure which story the first novel refers to the most.
Percy once again gets kicked out of school at the beginning, this time with his new friend Tyson, who actually turns out to be his younger Cyclops half-brother. Clarisse of the Ares cabin is granted a quest to search for the Golden Fleece; Percy and Annabeth aren't part of her quest but end up on the same trail in their search for Grover. Percy also discovers more of Luke's plan, due to both his dreams, and random luck. Luke is attempting to reassemble Kronos in a Golden Sarcophagus aboard a cruise ship, and he seems to be making quite a bit of progress.
I enjoyed the novel and am quite enjoying the series. I would say, however, that given the structure of the series this one is probably the one that feels most like filler - the first novel introduces the characters, the later novels in the series begin seriously facing the imminent danger while this one is more in the vein of "danger is rising, but we can't get too far into it." It's just like any season of Buffy or the Harry Potter series - the further you get, the more the episodes/novels focus specifically on the Big Bad rather than having loose tie-ins to the Big Bad or being side quests/side stories.