RICK RIORDAN's THE SEA OF MONSTERS is the second book in the PERCY JACKSON AND THE OLYMPIANS series. I'm reading these along with my son (age 10), and we both agree that this sequel is actually better than the first novel, THE LIGHTNING THIEF, which we also enjoyed.
As I noted in my post on The Lightning Thief, this series steals an awful lot from J. K. ROWLING's formula for the HARRY POTTER series. And yet, the theft is thinly disguised; Riordan doesn't try to hide it as much as make it his own. The differences between the series is telling. The chief difference is that Riordan borrows even more heavily from the Greek myths and manages, I think, to interest readers in returning to those old myths for a refresher course.
So far the stories are entertaining romps and I would recommend them highly as fare for young adults. I don't think they are all that memorable. I doubt they will be stories that the children of today will re-read as the adults of tomorrow, but not every book needs to be enshrined on one's shelf forever. Sometimes one just wants a good yarn, and the Percy Jackson stories, so far, fit the bill. Also, these are books that will get kids reading. My son disappears into them for hours.