For anybody who needs a kick in the rear to get involved in genuine christian community. This is the book for you. Bonhoeffer clearly shows the functions, duties, and incomparable benefits of being involved in deep relationships within the family of God.
Very thought-provoking, especially for somebody who is used to a modern evangelical church model.
Bonhoeffer is essentially a modern saint and his martyrdom showcased how he was a man who deeply practiced what he preached. Pick up and read anything he's written. He is both deep and simple, as well as philosophical and yet very practical.
Life Together is a series of 5 essays about various components to Christian unity. His first essay is on community, starting his essay by quoting Psalm 133, but ultimately basing it on Christ. His second essay is a treatise on the Christian family and his third is on the value of solitude and silence. The fourth essay is on how to prevent discord in community and then his final chapter is on the importance and how to practice confession.
This is a wonderful book, it's a short read but it takes awhile to digest what he's saying. It would make for a good small group resource (which would keep in the theme of Christian unity) and also falls under the general realm of spiritual practices/disciplines. If you've never read Bonhoeffer this would be a good start and if you have read some of his other works, this will not disappoint.