A real gem
Somewhere around 1998 I discovered the book The Theology of Joseph Ratzinger by Blackfriars Dominican Fr. Aidan Nichols. It's the best book on the development of Ratzinger's theology I'd ever seen (and have ever seen); unfortunately, it's out of print. Shortly after Ratzinger was elected pope, I went to buy it used via Amazon.com, only to find that it was going to cost me some forty dollars.
Fortunately, the publisher recognize that there just might be a market for the book, and so it was reprinted sometime in the last few months with a new title: The Thought of Benedict XVI: An Introduction to the Theology of Joseph Ratzinger.
Furthermore, Fr. Nichols informs us in a new preface that he will be coming out with a second edition of the text which incorporates the Holy Father's writings since the first edition was published in 1988.
If you're interested in the development of Benedict's theology, this is a must-read.