The series is a continuous story with plot threads and character development that builds throughout the series. Reading them out of order would spoil many plot points and leave you confused about the characters' motivations and relationships.
Here's the recommended reading order:
1. Fablehaven
2. Fablehaven: Rise of the Evening Star
3. Fablehaven: Secrets of the Dragon Sanctuary
4. Fablehaven: Grip of the Shadow Plague
5. Fablehaven: Keys to the Demon Prison
6. Fablehaven: Sanctuary (Prequel)
7. Fablehaven: Return to Fablehaven (Sequel)
Enjoy your journey to Fablehaven!