The message of this book is that reason, science and humanism—which Pinker identifies as the key themes of the Enlightenment—have, historically, led to massive progress in almost every area of life, and that they are our best means of continuing this progress into the future. But these ideals are not consistently upheld, and are often under attack. Therefore, we need to fortify and defend them.
This is an excellent book that I have been recommending to everyone I know. Just read it.
Relevant posts: Enlightenment Now: A summary