I have gotten a few laughs out of The Screwtape Letters, but it is also a particularly enlightening book. Here are some of the things that have stood out so far:
"There are two equal and opposite errors into which our race can fall about the devils. One is to disbelieve in their existence. The other is to believe, and to feel an excessive and unhealthy interest in them. They themselves are equally pleased by both errors and hail a materialist or a magician with the same delight."
"I wonder you should ask me whether it is essential to keep the patient in ignorance of your own existence. . . . We are really faced with a cruel dilemma. When the humans disbelieve in our existence we lose all the pleasing results of direct terrorism and we make no magicians. On the other hand, when they believe in us, we cannot make them materialists and skeptics." -Screwtape
"Do not be deceived, Wormwood. Our cause is never more in danger than when a human, no longer desiring, but still intending, to do our Enemy's will, looks rounds upon a universe from which every trace of Him seems to have vanished, and asks why he has been forsaken, and still obeys." -Screwtape
"I am almost glad to hear that he is still a churchgoer and a communicant. I know there are dangers in this; but anything is better than that he should realize the break he has made with the first months of his Christian life. As long as he retains externally the habits of a Christian he can still be made to think of himself as one who has adopted a few new friends and amusements but whose spiritual state is much the same as it was six weeks ago. And while he thinks that, we do not have to contend with the explicit repentance of a definite, fully recognized sin, but only with his vague, though uneasy, feeling that he hasn't been doing very well lately." -Screwtape