This book was a bit of a wild chase from end to end, which probably reflects the fact it is written for a younger age group than I usually read (I think it's still classed as YA, but probably the younger end). But the action was engaging and well-written, and the concept of Greek mythology alive and kicking in the modern world still works. Possibly better in this book than the first, since there's no need for as much explanation and backstory. 
 The story is another quest, as you'd expect, but different to the Lightning Thief to be interesting. I still have some problems with the reliance on coincidence - the amount of times Percy & his friends just happen across monsters at an inconvenient point unrelated to the overall plot, but it was both less frequent and less jarring in this book. The characters carry the book; they're likeable and you care what happens to them. They (especially Percy) do have a tendency to make stupid mistakes, some even verging on Too-Stupid-To-Live, but they do at least acknowledge when they've been foolish and try to learn from it. I also thought some of the minor characters came to life this time round, becoming more than the one-joke ponies they were in Lightning Thief. 
 A good book overall and despite my lukewarm feelings about the first book I'm glad I decided to continue with the series.