This is stimulating little book on preaching. Mitchell's two main goals are 1) For the preacher to be concerned with a singular behavioral purpose in each sermon. 2) For the preacher to be aware that the Holy Spirit operates on the intuitive dimension of the person and thus preaching needs to be celebrational (emotional), although he isn't advocating for an anti-intellectualism in sermons.
The most helpful things he does are: 1) provide a number of practical tips in this book especially suggestions for illustrations in narrative sermons. 2) Remind us that preaching and faith are not simply cognitive activities.
However, I don't agree with his theology of preaching. I definitely agree that humans aren't just cognitive, but I don't believe the Holy Spirit's work should be relegated to the emotional realm. Also, Mitchell has a deficient understanding of sin. Also, his philosophy of the sermon as having its main goal to create behavioral change misses the point. A sermon is proclamation so that people will see God. Once the listener encounters, it will lead to change. Mitchell believes that every sermon should end with celebration and be positive, but this doesn't allow the Bible to actually govern our sermons.