Lots of books are declared “unfilmable.” There are the big, tentacular genre epics, where the problem is scale and expense, like “Sandman” or “Dune.” There are your high-literary properties, whose tone feels too elusive (most of Don DeLillo, though people keep trying) or whose form is too baroque (“Infinite Jest”) to carry well into another medium. Then there’s the stuff that’s just too bleak to be commercially viable, at least in theory (“The Road”).