Whether you are a Christian believer or an atheist, (or just sitting on the fence) this book is essential. Along with the God Delusion and others, this is a powerful assault on the dangers and the irrationality of faith. It addresses both the problems of Christianity and rebutts the unfounded criticisms of atheism, with sobering reason and clarity. For atheists such as myself this represents yet another nail in the coffin of religious argument. Christians however, will find his tone measured and compassionate- he is appealing directly to their sense of reason rather than trying to upset them. It may not be as provocative as the works of Dawkins or Hitchens, but it gets straight to the point. Harris isn't interested in being controversial- he is just desperately concerned about the threat to civilisation that he believes religion represents in the United States.
This is also short book (or a long letter depending on your point of view), so even if your not a big reader, there is no good reason to ignore it. The world simply can't afford to.