...that one would probably be very hard to make into a good movie

There's no doubt that "The Last Battle" and "The Magician's Nephew" could be challenging to represent cinematically; but I don't think anyone would disagree that the two are critical to understanding the underlying allegory.

Only my opinion, but I think "difficulty in making the film" should not be the primary criteria for deciding whether to make the film. Using the "difficulty" argument, one could easily conclude that Tolkien's trilogy should never have been made. Instead, asking one's self which of the stories are core to C.S. Lewis's message and life's philosophy may be better guides in determining which of the 7 books to pursue.