The Divine Conspiracy (1998), though he wrote many other books as well. This book is an extended treatise on the Christian life, not as some ethereal or future thing, but as something earthy and present. Willard compellingly looked to the life and teachings of Jesus Christ as informing an actual way to live. He makes the argument that too often in modern evangelicalism, we look to Jesus as Savior, but have little hope that faith in him will produce any real sense of change.
The middle of the book is an extended treatment of the Sermon on the Mount, what Willard chose to re-title the "discourse on the hill." Willard looked to Jesus teachings in Matthew 5-7 not as a series of new laws for the Christian to follow but representative of the lives of those who are growing in Christ. In other words, those connected to the vine will increasingly manifest those things discussed in the discourse.
The second to last chapter is entitled "A Curriculum for Christlikeness". To me, this was the strongest part of the book. Here, Willard took all that he had previously discussed and applied it more directly to the life of the Christian. Known for his promotion of spiritual disciplines, it is here that he begins to discuss them with some detail, particularly the disciplines of silence, solitude, study, and worship. He makes a compelling argument for them.
The Divine Conspiracy is a good book. Willard's thoughts on the sermon on the mount were informative and worth pondering deeply. His defense of the disciplines too was useful. Having said that, I am never quite sure what to make of Willard. When I read his works or hear him speak, I often find myself wondering about his perspective on certain issues (e.g., is everyone saved?) because he never quite comes around to an answer. I also worry that some who make it as far as chapter 9 may see a causative, rather than correlative, link between disciplines and holiness that may not be quite so clear.
For the more academically minded, The Divine Conspiracy is worth reading, but for many readers, it will be too much. If you are interested in spiritual formation and growth in Christlikeness, but are not ready for a heavier work like this, it may be worth reading Willard's book Renovation of the Heart or even John Ortberg's The Life You've Always Wanted, which Ortberg himself referred to as Willard for Dummies, as both books are much more accessible.