I decided to read this as a follow-on from his excellent The Cost of Discipleship, which is in my opinion, one of the greatest works of Christian apologetics. Knowing Bonhoeffer's biography, it is obvious that this was his last work before he was murdered, and at times the retrospective knowledge that I had whilst reading it made me cry, especially when Bonhoeffer was hoping for a release in the not-too-distant future.
That said, the start is joyfully mundane, writing to his parents, requesting various reading material, how to keep fit in a prison cell and the joys of cigarettes. He moves on to more correspondence with his niece's husband, Eberhard Bethge, who later went on to be Bonhoeffer's biographer. There is a lot here which conveys much more of his humanity and compassion, along with recognitions of his own failings and foibles.
Some of the letters stand out more than others, and these tend to be when Bonhoeffer is questioning the status quo of christianity. He reveals that he was, in the true definition of the terms, a secular humanist, only without the atheistic connotations that we have come to associate with the phrase in recent years. His rejection of religiosity is something that his highly welcome although the evidence of this taking hold as popular thought until much later, with the likes of Rob Bell's Velvet Elvis being the modern expansion of this school of thought.
The only criticism I would have is in the translation. Bonhoeffer was fond of using latin phrases in his writings, but the translators have only included the english translation sporadically, so I had to keep looking up a lot of them, as they were not phrases in common use.
It is an immensely thought provoking collection and I cannot think of anyone I would not recommend this book to.