(Eberhard) Bethge did become Bonhoeffer‘s biographer, and dedicated the rest of his life to the works of his friend, without him, only a few traces of Bonhoeffer’s work be left to us. But Eberhard Bethge was much more than a biographer. The Letters and Papers from Prison that he published in 1951 contained only Bonhoeffer’s side of correspondence. Only when an enlarged edition appeared many years later, containing Bethge’s letters to Bonhoeffer as well, did we discover that Bonhoeffer developed his ideas in dialogue with Bethge. This was possible because they have been in ongoing conversation with one another since 1935. Thus, during the time Bonhoeffer was in Tegel, they could have said, like very different pair of friends, “I do my thinking in you, and do yours in me.”  

(Ferdinand SchlingensiepenDietrich Bonhoeffer 1906-1945: Martyr, Thinker, Man of Resistance, 348).