J.V. Fesko, Justification: Understanding the Classic Reformed Doctrine (Philipsburg: P&R, 2008).
At 461 pages, Dr Fesko manages to present the classical protestant doctrine of justification by faith alone, whilst interacting with opponents old and new. Although this is not a book on the NPP (New Perspective on Paul) Fesko thoroughly critiques NPP modifications to the doctrine of justification, interacting most notably with N.T. Wright.
Fesko starts out by examining the doctrine in Church history, before moving on to Prologemena, Protology, Exegetical theology, the links between justification and other doctrines (Union with Christ, Sanctification, the Final Judgement, and the Church), and finally examines the differences between the doctrine of justification by faith alone and the teachings of both Roman Catholicism and Eastern Orthodoxy.
This is not a nice, simple introduction to the doctrine of justification; it would be amiss of me not to warn you of that. This is not the book I'd be recommending to the average person in the pew who wants to know about justification (although Fesko's other book on justification would be ideal for such a purpose). Rather, it is a book for those who already know the basics of justification as 'an act of God's free grace, wherein He pardons all our sins and accepts us as righteous in His sight, only for the righteousness of Christ imputed to us and received by faith alone.' (WSC 33). As Michael Horton comments on the back cover, 'this book is destined to become a standard work for all who want a deeper understanding of this crucial doctrine.' If I were teaching a class on Soteriology, I would make this book required reading. For theological students and ministers this is a great book to read.
One of the great advantages of Fesko's book is that it is so up to date. While there are many good older books on the doctrine of justification, they do not deal with some important issues that are currently of great importance, namely issues related to the NPP. While a spate of books addressing the NPP have been written in recent years, they do not give full-scale treatments of the doctrine of justification. Fesko combines the best of both worlds. The way that Fesko keeps both the historia salutis and ordo salutis in view throughout his work is also a great strength.
If you want a monograph on justification, this is it.