A good friend recently mentioned that he had just finished the audiobook version of Childhood's End by Arthur C. Clarke. (Highly recommends it, by the way.)
I find it somewhat surprising that no film version of that novel was ever made. Both Childhood's End and 2001: A Space Odyssey begin with pretty straightforward near-future Science Fiction settings and trundle headlong toward seriously mystical, eschatological stuff in their endings. I don't know why one would have been filmworthy and the other not.
Could it be because studios didn't think the idea of an invasion by benevolent aliens would sell? Or, maybe they were leery of those creepy telekinetic kids who emerge as mankind's ultimate generation.