While A Testament of Devotion comes in a close second, the book that has been most helpful to my life of discipleship is Life Together by Dietrich Bonhoeffer. First off, of all of the people in Church history, Bonhoeffer is the person whom I most admire. His commitment and willingness to side with the marginalized and eventually become marginalized himself has both inspired me and kept me living in reality (at least I think so!), something that is not easy to do in the midst of a consumer culture and so much affluence.
Even so, Bonhoeffer is known for many important books—The Cost of Discipleship, Letters and Papers from Prison, etc.—so what makes Life Together stand out?
At least two aspects. First Bonhoeffer's clarity considering the possibilities for and limitations of human relationships. He writes, "Spiritual love . . . comes from Jesus Christ; it serves him alone. It knows that it has no direct access to other persons. Christ stands between me and others." For Bonhoeffer, love without God, self-centered love, is interested only in what the other person can do for me. Love in and through Jesus Christ sees the other as God sees them, a person that is not here as a means for my ends but as a God-bearer, here to contribute to the redemption of the world.
Second, the balance between the contemplative and active life that Bonhoeffer sees as necessary for a community life together that is functional and real. He refers to these in different sections of the book as "The Day with Others" and "The Day Alone." No matter in what type of community we find ourselves, even if we live alone, this is helpful guidance as it encourages us to be in the midst of others and to take time apart. This is both for the sake of ourselves and for the sake of others. Yes, we need to spend time alone, in his words "The Day Apart," for the good of other people, that we might regain our perspective and consider how our lives are impacting those around us. Yes, we need to spend time together, "The Day Together," for the encouragement of others and to help us better see the ways we live in self-deception. As Bonhoeffer puts it, ""Let him who cannot be alone beware of community . . . Let him who is not in community beware of being alone."
Having lived in intentional community for a number of years, this book was of great assistance in helping me know how best to encourage others. I do think the potential application is broader, to our relationships generally.