Mere Discipleship by Lee C. Camp has been a challenge for me and will probably continue to be. Camp does a great job of identifying a "cataract" on Christianity that goes back as far as Constantine. The cataract more often causes us to approach our Christian walk through the eyes of nationalism rather than through Jesus himself. Often our approach to issues, such as war, are influenced through politics, capitalism, and decisions made earlier in history.
In other words, we may not be following Jesus first. We may be following an assortment of other influences that say they are following Jesus too. I can't do this the same justice as Camp, so I'd read his book.
If you are a member of a Church of Christ or have ties to Lipscomb University in Nashville, then you may have a few things in common with Camp. He's very sharp.