Well... it has been made in to a film, but not in the same way things like The Da Vinci Code, Narnia, Hitchhikers and whatnot have.
Have you seen a film called The Hours? It's based on the Virginia Wolfe book "Mrs. Dalloway", but instead of being the same plot as the book, it shows how the book has affected the lives of three women from different periods. And there are references to the book, and slight snippets of the plot. I imagine that's the same thing they've done with Finding Holden/CitR.
Absolute Power was a GREAT book, but a pretty weak movie. That is definitely the trend with good books gone to the big screen. I actually thought they did The Da Vinci code pretty good. It was basically spot on with the book, which I liked.
It might have been spot on with the book, but it moved far too fast. I didn't think the script was too good, either - a lot of stating the obvious went on.