Bernie Glassman is one of the greatest American Zen teachers. In this book, he goes deeply into some core teachings of Zen. He starts with the Heart Sutra (Prajnaparamita Hridaya Sutra). When I started attending a Zen temple, the nightly chanting of this sutra always confused me, but Glassman's line-by-line analysis shows how this sutra contains great wisdom, and makes understanding clearer.
His analysis of the precepts has been the most helpful to me. He explains the various ways of interpreting each precept, and demonstrates the need for balance. For instance, one of the precepts is non-killing. Some sects of Buddhism extend this to not killing insects and micro-organisms. Glassman explains that to understand this precept, we need not follow it to this level, but we must be aware of it, and try to cause less damage. The precepts, on some level, are an admonition to try our best, and Glassman's discussion will help us to be peacemakers in each moment of our lives.