Maybe trite, but the Bible. For a mind-blowing experience, read Matthew chapters 5–7, and have in mind that the topic is: “what it means for a person to be righteous, or good.” The analysis of the role of moral rules, the place of worry and anxiety in driving us to short-sighted compromise, the tension between “secret” good-doing versus good-doing for public recognition, are all potent, helpful, and life-changing.
It made me turn from an doubtful agnostic theist into an agnostic atheist 1/4 through it.
Amazing read. Will definitely not read it again!