From Screenhead comes a list of the hardest novels to film. Included in the list are, Ulysses, 100 Years of Solitude, and Catcher in the Rye. If it were up to me, I'd also like to add Kerouac's On the Road. A film adaptation has been in the works for years, I once heard Brad Pitt was slated to be involved but I think that ship has sailed and who knows if it will ever really be brought to the screen. What book would you consider unfilmable?
Read about The Unfilmables.