This book by Cardinal Ratzinger, the new pope Benedict XVI, is an exploration of the philosophy of religion. As such, it is an important guide to the philosophical underpinnings of the theological statements made by Ratzinger while he was Prefect of the Sacred Congregation of the Doctrine of the Faith (one of the 'high offices' of the cardinals in the Vatican), and the kinds of philosophical guidelines he is likely to follow in the future.
He explores the construct of religion and religious truth from an historical perspective, drawing from three primary strands - ancient mysticism, the development of monotheism, and the Enlightenment. What we in the West live in today is a post-Enlightenment world, with monotheistic tendency in religion, with continuing strands of mysticism that often impact society in unpredictable and uncomfortable ways.
With regard to monotheism, he explores through different religious traditions the way in which this concept can be played out in culture. In terms of the Enlightenment, he explores philosophical antecedents in Plato and Aristotle carrying forward through the Phenomenologists of the early twentieth century. This is where Ratzinger's academic strength lies - in philosophy and theology.
Ratzinger also looks at the different ways in which Christian perspectives of the idea of philosophical and theological truth are seen in other religions, including perspectives can lead to the idea of the anonymous Christian (a Rahner-ian concept, often termed inclusive or pluralistic, depending upon the details), as well as an exclusivity standpoint - this is not where Ratzinger ultimately comes down in terms of philosophy.
This book is derived from lectures and sermons, it is clear. There are times when it seems to be survey, and times when it seems to be more argumentative/persuasive. Because it is an academic text, it provides a broader range than a papal encyclical might, but it is still a good resource for exploring the insights of the man now Pope Benedict XVI.