‘The Screwtape Letters’ by C.S. Lewis
I often argue that religious morality has a deep advantage over secular morality. Both look similar from the outside, but religion continuously spurs a person to self-examine inner motivations and honestly assess their own character in a way that the secular stimulus of societal censure cannot manage alone.
C.S. Lewis examines this ‘inner life’ through the imaginary medium of two disgustingly-named devils strategically conspiring to win a soul for Hell. It works well. He lays bare the little tricks that we use every day to disguise our true motivations even from ourselves. We are proud, for example, of our ‘unselfishness’ – which is really the exact opposite of the selfless love that should be the only reason for acting.
It is a witty book too, which is perhaps what saves it from a descent into bland sermonising.