The Lightning Thief

Megan and I read this book together. There was much action, and much backstory. The movie version was very good at getting just about all the action scenes to come to life. But it simplified the backstory, which made sense onscreen. The movie was very straightforward: 4 distinct "missions", one twist at the end. The movie did not have the god Ares (god of war) involved at all. Hades is rather evil in the movie, where in the book he turned out to not want war at all. The special effects were very good... enough to scare Cameron away from the screen. Percy is at least 4 years older in the movie (probably 16 compared to 12), but, as David said "no one wants to watch a 12-year-old hero." That might be true.

Percy's mom did not kill Gabe with Medusa's head, but she did kick him out. Also, there was a romance implied between Annabeth and Percy, and that did not really come out in the book. Maybe in the later stories, though.