Page 17 states the central concept very well: "This seems paradoxical: the finite contains the infinite. But it is God Himself who made this possible, by assuming human flesh in Jesus Christ. In doing so, He humanized His divinity, but He also divinized humanity, and thus sanctified - made holy - everything that fills up a human life: friendship, meals, family, travel, study, and work."
Thus, Opus Dei is about the sanctification of all aspects of our daily lives. Through a deep prayer like that includes the Mass, Rosary, Liturgy of the Hours, etc along with doing all things in great love, we grow closer to God. For Jesus taught us an example to follow, and we should realize that He too worked continuously to proclaim the Gospel. Work is something that can be made holy.
This book is highly recommended to those interested in Opus Dei. I have read both positive and negative things on Opus Dei. Please do not base all of your opinions about the organization solely on this book. Several books concerning Opus Dei have already been added to the comment box below by readers. I suggest reading other books on the subject before coming to a conclusion on the organziation.
Note: This book has a Nihil Obstat by Reverend Michael F. Hull, STD, Censor Librorum. It has an Imprimatur by Most Reverend Robert A. Brucato.