This text in particular demonstrates through the Bible that God has always had a concern for the cities his people live in. It makes the point that, as people from all over the world flood into our cities (in fact half the people in the world live in cities), cities are the new mission field.
By the way, what comes to mind when you hear the words "mission field"? For many, it takes courage to say it, because it is unpopular work. The untrained and the spiritually weak are too scared to acknowledge that the work needs to be done. That's where TUMI fits in. Their teaching clarifies the necessity, while their training instills the skills, confidence, and knowledge to get it done.
Each of the supplemental books I've read so far have been awesome. They say the education we're receiving from them is seminary-level, but I think that much of this knowledge should be dispersed to every adult who is a professing Christian.
From the time I entered middle school, my faith became a target for the world to ridicule. You know it wasn't just me. I saw other kids getting ridiculed for the 'turn the other cheek' teaching and I heard kids of other faiths repeat the challenges to our faith that they heard from their parents.
We need answers to those challenges, either before or shortly after those attacks come, or it stunts our spiritual growth. Fortunately, it seems that finding the right answers at any age puts us back on track.