The Four Agreements
by Unknown
Ruiz draws on Toltec wisdom to distill an entire philosophy of living into four rules: be impeccable with your word, don't take anything personally, don't make assumptions, and always do your best. The simplicity is the point, because each agreement, when you actually try to practice it for a week, turns out to be far more demanding and revealing than it sounds. Not taking things personally, for instance, requires you to genuinely accept that other people's behavior is about their own reality, not yours, which is a radical shift for most people and one that eliminates an enormous amount of unnecessary suffering. This is the shortest and most accessible book on the list, and readers who come to it skeptically tend to be the ones who benefit most.