In the opening pages Bonhoeffer writes against believing in a cheap grace. Grace is costly, he argues, and therefore, so is discipleship. There's no cheap price tag on true discipleship. No discounts are offered. No short cuts have been discovered.
Discipleship is much more than a course to be mastered and a grade to be earned.
Certainly, we must study what it means to be a disciple. The content of the Sermon on the Mount must be mastered if we are to live it. Journals or notebooks and other tools can help us in developing our personal spiritual discipline. But being a true disciple is a total life-style. Having learned something of what it means to be a disciple doesn't mean we have arrived.