In reading order
39 Clues: Cahills vs. Vespers
Gordon Korman
A King's Ransom (The 39 Clues: Cahills vs. Vespers, Book 2)
Jude Watson
The Dead of Night (The 39 Clues: Cahills vs. Vespers, Book 3)
Peter Lerangis
Shatterproof (The 39 Clues: Cahills vs. Vespers, Book 4)
Roland Smith
Trust No One (The 39 Clues: Cahills vs. Vespers, Book 5)
Linda Sue Park
Day of Doom (The 39 Clues: Cahills vs. Vespers, Book 6)
David Baldacci