Volume 5 of Dietrich Bonhoeffer's works which contains Life Together and Prayerbook of the Bible. This is the first time I've read anything by Bonhoeffer and overall I thought both works were excellent.
Life Together deals with the concept of Christian community and how Christians should live in community. Bonhoeffer emphasizes that true Christian community can only happen through union with Christ. We are all called to live in Christian community but we will only rarely "experience" Christian community. Often true Christian community is something we confess by faith rather than something we experience. Bonhoeffer provides very helpful advice in regards to different approaches to Christian community. Unfortunately, some of this advice is stated as if it were a direct command from God.
Prayerbook of the Bible is about the importance of using the Psalms to guide our prayers. Like the church through most of history, Bonhoeffer takes a Christocentric approach to the Psalms. This approach is very helpful, especially when praying the imprecatory Psalms or psalms where we are asserting our own righteousness. Even when we pray Psalms acknowledging our own sinfulness, Bonhoeffer notes that Christ is the sin-bearer.
Both works have lots of notes provided by the editor, both explanatory and references for further research. There are very good introductions provided by the editor that place the works in their historical context and provide lots of good background information.