The Five Most Fantastic but Unfilmable Books
Computers changed all that of course, making the filming of impossible imagination a budgetary constraint rather than a physical impossibility. But even as recently as the nineties, computers weren't the answer to everything. Hell, James Cameron filmed Titanic by building the freaking Titanic and sinking it again. It was only 2/3 size instead of full size simply because of something about camera angles.
We are teetering on the edge of being able to create absolutely anything in photorealistic detail, provided the artist or filmmaker has the skill to actually pull it off. That doesn't mean all CGI looks great, or even should always be the tool used, anymore than the existence of dictionaries and word processors means we're all Shakespeare. But it does mean that the barrier of what makes a story unfilmable moves away from technical ability and into the actual strengths of the medium. There are stories that simply cannot be done justice in the form of film.
5. Ender's Game: Yes, I know that there is a version finally in active production. But while a film will certainly be able to capture the events of the book, I have almost no confidence that it can capture what made the novel so captivating, and I don't just mean that CGI can't make twenty pre-teens competent enough actors that we become convinced they are geniuses. I mean that the action of the novel all happens in Ender's head. The empathy, the struggle, the quiet watching and learning. That invisible internal struggle cannot be directly filmed, and I don't see this story retaining its greatness without it.
4. Dune: A similar problem arises with Dune, a story that has already been the subject of a film and a mini-series. The technology is there to display the events. But the internal struggle of Paul with seeing the future, the constant tip-toeing along boundaries between what he has foreseen and what he hasn't, that is not something visual that can be conveyed any better than it is with words.
3. Night and any other book about the Holocaust or similar horrors. The words and their poetry can give some comprehension, but the films limit by capturing too much. Words by their pressure that forces images without literally drawing them, have an element of the infinite that are checked in film. As if the very act of literally portraying something of infinite horror gives it definition, gives it boundaries, allows some form of rationalization.
2. Discworld: There have been individual television films made from some of Pratchett's novels, but the drawback of them is that they will always be incomplete. Discworld stands as one of those monumental achievements of literature: forty novels, all set in the same world, with interconnected stories and characters lasting decades. In order to do this right, you wouldn't even be able to do a television series, there is that much intertwined content. Tell one story and half of it will be dependent on previous stories. Tell the early stories and you lose all the richness of the later ones. Sheer size becomes a limitation all its own.
1. Sandman: As an HBO series? Not even then. Although as a comic book, there is of course a visual element to the magnificent series, it is not a literal interpretation of events. Those drawings and snapshots that fall short of photo-realism invite the mind to fill in the blanks, and so much of the action in Gaiman's master work takes place in those gaps between pictures, in the universe of images that the insufficient ones on the page spark in the imagination. Films can capture something like that, but I don't think that they can capture it in exactly the same way that these comics do. Any adaptation would be a true adaptation, a re-interpretation that is not exactly the same work of art as the original.