More like 3.5 stars. I had a difficult time deciding between the two, but the story is more fun, and a bit too simple to deserve 4 stars. Of course, it's more for a younger demographic, still, it was a good light read.
The story itself is clever, but not too inventive. It can be recommended to young people as an introduction to the Greek pantheon. In a way it's very educational, and having always been a fan of Greek mythology, I can appreciate that aspect. It also does a good job of making it interesting for contemporary people. However, since most of the mythology is not invented by the author, I deducted points for it.
The story itself has a good pace. The language is easy to read, not overly descriptive, but still gives a good sense of the places and the people. It focuses on adventure, and since I'm not a fan of overly flowery language, I appreciated the simplicity. I gobbled up the whole thing in a few days.
It's also educational in that it often stresses the visible reality of pollution. It's interesting to note that at one point there is also a mention of not just physical, but mental pollution, that is done by negative thoughts.
I could empathize with Percy Jackson. He had ambivalent feelings about his father, and had to work through some issues. His devotion to his mother was also understandable. I have a similar relationship with my parents, therefore I could identify with his character.
Annabeth was a bit understated. She was the girl in the team, and I felt she was often forgotten. She was the brains of the operation, but I felt she should have had a more important role. Hopefully in future books.
Grover was a lot of fun. In a way, his character was the comic relief, but couldn't be completely dismissed as such.
Overall, the book is more for the 12-14 demographic, however, can be appreciated by adults as well. It should not be taken seriously, but as a light adventure, a bit of fun at the end of a hard day. For younger people, it's a good teaching tool for ancient mythology, as well as environmental protection.