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Letters is an epistolary novel and those are immediately troublesome as far as adaptations go (there are exceptions of course. There always are). In a film version they will have to actually dramatize events you're only hearing about in the letters of the novel. These events are told through a skewed perspective, the demon's. Part of the joy and the strength of the novel is reading the words and reinterpreting them. A movie will also have to visualize hell, senior and junior demons and the like and immediately rob the powers of suggestion of the book --it's not big on visual descriptions, being a very conceptual thing. This kind of thing can only end one way: corruption and over simplification of the brilliant source material. I'm horrified that the Lewis estate would even consider selling the rights to this.