The Bonhoefferian is a blog devoted to the theologian. In contains some good reviews and articles, including a link to an excellent article “Ten Theses on Dietrich Bonhoeffer: Christian, Theologian, Martyr” in Faith and Theology and notice of an International Conference on Bonhoeffer in Prague, July, 22-27, 2008.
Another new blog is posting excerpts from Bonhoeffers letters.
If you want to learn more about the history of the period, Kansas State University offers an internet course on “The Rise and Fall of Nazi Germany.”
In English
Discipleship (Works, volume 4) is a New Translation of Nachfolge, with a simpler title. The older translation was The Cost of Discipleship. This new edition is “freshly translated from the German critical edition, Discipleship provides a more accurate rendering of the text and extensive aids and commentary to clarify the meaning, context, and reception of this work and its attempt to resist the Nazi ideology then infecting German Christian churches.”
The massive biography by Bonhoeffer’s best friend (the recipient of most of the letters) Eberhard Bethge, is the authoritative source, Bethge’s lifelong labor of love. Amazon. For a series of reviews of the chapters, see “Blogging Bethge.”
Und auch Auf Deutsch:
The German edition of the “Letters and Papers.” Amazon.de
Nachfolge, his classic book on discipleship, new edition of 2002. Amazon.de
Bethge’s biography auf Deutsch, all 1140 pages. Amazon.de
Theological German/Theologisches Deutsch 
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