There should be a movie for each book (including Cahill Files) but as we all know, that's most likely not going to happen. But maybe if they were to do it like this:
The 39 Clues: The Movie - Books 1,2, and 3
The 39 Clues 2: In Too Deep - Books 4,5, and 6
The 39 Clues 3: The Viper's Nest - Books 7,8, and 9
The 39 Clues 4: Into the Gauntlet - Book 10
The 39 Clues 5: Vespers Rising - Book 11
The 39 Clues: The Cahill Files - CF Books 1 and 2 (Released in double DVD set with Vespers Rising as short stories)
The 39 Clues 6: A King's Ransom - CvV Books 1,2, and 3
The 39 Clues 7: Shatterproof - CvV Books 4 and 5
The 39 Clues 8: Day of Doom - CvV Book 6
The 39 Clues 9: Nowhere to Run - Unstoppable Books 1,2, and 3
The 39 Clues 10: Unstoppable - Unstoppable Book 4
10 movies, each with 2 or 3 books (with the exception of Into the Gauntlet, Vepsers Rising, Day of Doom, and Unstoppable). Then a bonus The Cahill Files released as a double DVD pack with Vespers Rising. What do you think?