I see that it will be harder to make more than one movie of the 39 Clues because of the fact that the actors will age. I can see ways around that, though. But I wouldn't get all worked up if they decided to do one movie. I would suggest that a trilogy would work, I guess, for the first part of the book series. Like if for the first movie the actor for Dan was 10, but played an 11 year old, and the second movie the actor can be actually 11, and the third movie he can be 12 playing 11.
If there was on movie per book (the first series, 10 books) over the course of 10 years, the actors and actresses would age 10 years....what!?! Unless they changed actors. The fact that Logan Lerman was 10 years older in the Percy Jackson movie than the Percy Jackson book ruined it for me. But maybe there's just something wrong with me. But you have to admit the story was very different than the book, but similar enough to be considered based on the book.
The Diary of a Wimpy Kid movie also dosen't have one movie per book, because then Greg would be 20 by the eigth book!
It hope that because of Spielberg producing it, it will be a good film. And, taking into account that the younger actors/actresses will noticebly age, a TV series would be great.