These are my top recommendations for those interested in learning more about Dietrich Bonhoeffer:
Dietrich Bonhoeffer: A Biography, by Eberhard Bethge. Bonhoeffer's original biographer and close friend.
Dietrich Bonhoeffer 1906-1945: Martyr, Thinker, Man of Resistance, by Ferdinand Schlingensiepen. A shorter "update" of Bethge making use of more contemporary research.
Bonhoeffer - Agent of Grace [VHS]: A PBS documentary
Bonhoeffer, another movie, this one with direction and footage with friends and family members.
Life Together and Prayerbook of the Bible (Dietrich Bonhoeffer Works, Vol. 5): This is the fifth volume in Fortress Press's magisterial complete works of Bonhoeffer in English. It might be the most accessible volume, LT is about life together as a seminary at the confessing church seminary Bonhoeffer led. The volume also includes his commentary on the psalms. This may be a bit more approachable than his more difficult, if more famous, Discipleship (Dietrich Bonhoeffer Works, Vol. 4)
Bonhoeffer's theology developed over time, and it is worth reading his late work, especially Letters and Papers from Prison (Dietrich Bonhoeffer Works, Vol. 8) or Ethics (Dietrich Bonhoeffer Works, Vol. 6). The first is a book assembled by Bethge after Bonhoeffer's martyrdom. The second was the last book he worked on (but did not have the chance to finish) in the final years of his life. Both are seminal, and are where I will be devoting my reading energy in the next couple of months.
Note that I do not list Eric Metaxas's Bonhoeffer biography here--although it is on the best-seller list, I consider it to be too full of historical inaccuracies, and an attempt to make Bonhoeffer something different than he really way. Bethge and Schlingensiepen are much more solid choices in biographies.