I found this book to be a bit simpler than The Lightning Thief, though I suppose that's mostly because the exposition isn't necessary. We get much less about Percy's mother, less backstory, less mystery. Instead, Sea of Monsters is a very surface-level quest narrative: Percy arrives at Camp Half Blood, is charged with finding the Fleece, encounters a few perils on his way, finds it, comes home, the end. It felt very pared down, and rather short. 
 Don't get me wrong, it did work as an adventure story, it just felt very straightforward. I always enjoy the way Riordan "modernizes" Greek myths, and Circe's island was particularly interesting. 
 And the book does move the overall plot forward. The reveal at the end, which I won't spoil, sets up a very interesting storyline. But it took a whole book to get there, and the story did feel very empty. I had fun, but it wasn't very filling.