McGrath's History of Justification
I recently read Iustitia Dei: A History of the Christian Doctrine of Justification (Third Edition) by Alister McGrath.
Overall, it's not a book I'd recommend. The book runs to 400 pages and deals in detail with the thinking of medieval and reformation scholars on grace, the justice of God, and the process of justification. The two major things I got out of it were that:
1) Everyone prior to the Reformation saw "justification" as involving actually being made righteous. The Protestant ideas of forensic justification and distinguishing justification from regeneration and sanctification were fundamentally new and unprecedented in church theology.
2) The pre-Augustinian fathers didn't have much interest in the word "justification" and talked about salvation using other ideas. In the last 300 years the Roman Catholic church has largely stopped using the word also, and in the last 50 years most Protestant churches have largely stopped using it too.
There, I just condensed a 400 page book to two small paragraphs...