Some books are totally filmable. I wouldn't put The Hunger Games on this list because other than an issue with some first-person point of view storytelling, the book was easily imaginable as an adaptation. I think we've reached a point where very little is unfilmable because of budget or effects, the only impediment being length of story (you wouldn't want Game of Thrones in two-hour increments for each book). We have a number of films to judge against their source material in the near future - Life Of Pi, Anna Karenina and The Great Gatsby each come with unique obstacles. I'm curious to see how a director attacks the problem of a lack of consistent narrative story in Kerouac's On The Road.
There are a few on my list off my library shelves that I still can't imagine on the silver screen. Among them is Catcher In The Rye, a book with a built-in audience but hard to imagine finding a plot to hold onto. In the graphic novel category, Y: The Last Man is a good complete story that would require much more than a single sitting to let the characters come to life. In novel form, The Story of Edgar Sawtelle is intriguing but films from a dog's point of view are few and far between. The ultimate request would be for an adaptation of Sandman but Neil Gaiman's books have seemed untouchable in the past. I don't feel like we are limited by portrayal of the Dream world anymore but each story is so dense, I can't see it happening except as a series of mini-series.
Directors will continue to accept the challenge to make films of books they love. It's a testament to the continuing power of the written word. How can directors recreate the theater of the mind? They can't if you don't follow my one mantra - read the book first.