The Silmarillion?
My thought as well. All of it would be impossible, it's too disjointed, but perhaps the story of Beren and Luthien and the fight against Morgoth? Or perhaps Isildur's story from the Second Age? You could probably get a movie out of either but there isn't a great deal of material for a screenplay.
I can't see how that could be done in less than....say....twelve parts or so.