“I was often asked or advised to add to the original Letters, but…I felt not the least inclination to do it. Though I had never written anything more easily, I never wrote anything with less enjoyment. The ease came, no doubt, from the fact that the device of diabolical letters, once you have thought of it, exploits itself spontaneously…. It would run away with you for a thousand pages if you gave it its head. But though it was easy to twist one’s mind into the diabolical attitude, it was not fun, or not for long. The strain produced a sort of spiritual cramp. The work into which I had to project myself while I spoke through Screwtape was all dust, grit, thirst, and itch. Every trace of beauty, freshness, and geniality had to be excluded. It almost smothered me before I was done. It would have smothered my readers if I had prolonged it.”
Also I found in this Preface the "Lewis treatment" of the claim that God and Satan are direct opposites. For Lewis, this question really amounts to the formulation: “Do I really believe in the Devil.” His answer:
“….if by ‘the Devil’ you mean a power opposite to God and, like God, self-existent from all eternity, the answer is certainly No. There is no uncreated being except God. God has no opposite. No being could attain a ‘perfect badness’ opposite to the perfect goodness of God; for when you have taken away every kind of good thing (intelligence, will, memory, energy, and existence itself) there would be none of him left.
"The proper question is whether I believe in devils. I do. That is to say, I believe in angels, and I believe that some of these, by the abuse of their free will, have become enemies to God and, as a corollary, to us. These we may call devils. They do not differ in nature from good angels, but their nature is depraved. Devil is the opposite of angel only as Bad Man is the opposite of Good Man. Satan, the leader or dictator of devils, is the opposite, not of God, but of Michael.” [Bolds added for emphasis.]
I've always found C.S. Lewis' theology to be morally satisfying and sound, especially for beginners. It's very hard for me to believe in a loving God who would condemn us to hell before we really had a chance to figure out what we were doing. So the idea that hell is a place we can leave if we choose to let go of our sins is appealing to me.