Let me re-explain my last query from Tuesday. I didn't mean to suggest that all important novels are unfilmable—see Great Expectations, etc. I meant to point out that calling novels "unfilmable" is great gambit for studio execs, whether they actually believe it or not. It allows you to dump on pictures you don't like—or don't have a stake in—by flogging the source material. (Louis Mayer might have honestly thought Red Badge had "no story." But it wasn't his picture, so we can never be sure of his motives.) And it's a no-lose situation. If the movie flops, as in the case of Red Badge, you can say that you predicted it all along. If it's a hit, as with L.A. Confidential, nobody remembers what you said anyway.
I took the two most recent examples you mentioned—L.A. Confidential and The English Patient—and plugged them into the Nexis search engine with the word "unfilmable." It brought back more 120 combined hits, meaning that many studio execs had no confidence in those titles, either.
This is very similar to the strategy used by studio marketing bosses, as Art Linson explains in What Just Happened? After seeing a movie for the first time, the marketing chief pronounces it unsellable, regardless of its quality. That way, if he manages to sell it, he's a genius. If not, well, he warned them.
What did you think about The Devil's Candy?