The one drawback to this book is that at times it reads as the work of a 21st-century conservative American evangelical (which is what Metaxas is) assessing Bonhoeffer's life, rather than a timeless biography.
Nevertheless, I got a lot out of it, not previously knowing anything about Bonhoeffer.
Bonhoeffer was a theologian whose distinctive idea was that a Christian church must be proactive rather than reactive in the culture of the day. That was a viewpoint that, it seems, both informed his decision to oppose the Nazis and conspire against Hitler and in turn was shaped by the rise of the Nazis -- a regime totally at odds with the Christianity that Bonhoeffer knew.
The clearest example of Bonhoeffer's theology informing his actions was his early and strong stance in favor of protecting the German Jewish community from the Nazi regime. That stand was important in the split between Bonhoeffer's church and the state-sanctioned Christian church that arose during the Third Reich.
Bonhoeffer is an example of someone who retained moral clarity despite finding himself on the wrong side of history and his fellow citizens. Metaxas' account makes clear that Bonhoeffer had no illusions about Hitler and the Nazis and what their leadership represented, even as other people of good will struggled to understand the threat that Nazism posed to society in time. As Nazism rose, he progressed from conciliatory, to defiant, to conspiratory, remaining ahead of others both in and outside Germany at each stage. Definitely a figure worth learning about.