Here's why:
* Storyline progression: The series is a continuous narrative, with each book building upon the previous one. There are recurring characters, developing relationships, and a complex plot that unfolds throughout the series.
* Character development: The characters, especially Percy himself, grow and evolve over the course of the series. Reading out of order will disrupt their journeys and the impact of their choices.
* World-building: The series introduces a unique and intricate mythology-based world. Each book expands on this world, introducing new gods, monsters, and locations.
* Surprise and impact: Many important events, twists, and revelations are directly tied to events from previous books. Reading out of order will spoil these moments and diminish their impact.
Here is the recommended reading order for the Percy Jackson series:
1. *The Lightning Thief*
2. *The Sea of Monsters*
3. *The Titan's Curse*
4. *The Battle of the Labyrinth*
5. *The Last Olympian*
Note: There are also several companion books and spin-offs in the same universe. You can explore those after finishing the main series.