Here are their titles:
1. The Maze of Bones
2. The Sword Thief
3. The Cahills vs. Vespers
4. The Secret of the Old Clock
5. The Phantom Tollbooth
6. The Emperor's Code
7. The Black Circle
8. The Conspiracy of the Caves
9. Beyond the Grave
10. The Serpent's Shadow
There are also several side books that expand on the main series, including:
* The 39 Clues: Case Files
* The 39 Clues: Unstoppable
* The 39 Clues: The Viper's Nest
* The 39 Clues: The Last Sacrifice
Let me know if you'd like to know more about any specific book or series!