This month's free audio book from ChristianAudio.com is The Cost of Discipleship by Dietrich Bonhoeffer. Here is the description from the ChristianAudio.com product page:

This book is quite simply, one of the most profound and important books of the 20th century. Dietrich Bonhoeffer lived a testimony of his thoughtful and engaging writers.

Focusing on the most treasured part of Christ's teaching - the Sermon on the Mount with its call to discipleship, and on the grace of God and the sacrifice which that demands.

Viewed against the background of Nazi Germany, Bonhoeffer's book is striking enough. At the same time, it shares with many great Christian classics a quality of timelessness, so that it has spoken, and continues to speak powerfully, to the varied concerns of the contemporary world.

Warning: Bonhoeffer was known to have been influenced by Neo-Orthodox theology, so be alert to such influences, particularly for a weak view of the inspiration of Scripture.