The point of the book, in a nutshell, is to moderately push back against NPP caricatures of Reformation theology while also providing an in-depth examination of what, exactly the Reformers (especially Luther, Calvin, and Melanchthon) believed about Pauline theology.
The key term here is "exegetical grammar," i.e., what Luther meant about the term "justification" compared to his Catholic opponents, etc. Chester focuses a lot on the debate between Luther and Erasmus, including how they appropriated Augustine differently.
The last section of the book focuses on facilitating a dialogue between between a theology of the Reformers and the New Perspective on Paul. To be clear, Chester does not give the Reformers a free pass, and offers clear criticism of Luther, etc., when necessary. However, he generally argues (fairly, I believe) that the NPP has misunderstood Reformation Pauline theology; thus, for Chester, the NPP has "thrown out the baby with the bathwater," so to speak.
I believe Chester has written an extremely important book that offers an appropriate but irenic corrective to some of the excesses of the NPP. This book helped me understand both Reformation theology and the NPP much better. The one main downside of this book is that if historical theology, specifically Reformation-era theology, does not interest you, it takes a long time to get to the point where Chester dialogues directly with the NPP, and so some readers might lose interest. Those readers only interested in a critique of the NPP should skip to chapter 8.
For the interested reader: the best and worst of the NPP is, I believe, adequately summed up by Chester on page 361:
"The NPP does represent a very significant advance in its portrayal of Judaism. Former descriptions of Second Temple Judaism as a religion centrally concerned with earning righteousness were a distortion and the exegesis of the Reformers lay at the historical roots of this distortion. Their interpretation of Paul's contrasts between the law and his gospel almost exclusively in terms of self-achieved works-righteousness is unconvincing. . . Yet NPP scholarship simply perpetuates the opposite error. A theoretical acknowledgement that the phrase the 'works of the law' denotes the whole complex of conduct required by the law is coupled with an actual insistence that Paul's concern is always with the boundary-defining function of such works. This boundary-defining function is indeed important, but the exclusive emphasis upon it does not do justice to the multi-faceted and all-embracing nature of the conduct required by the law. . . . To say that justification does not result from human ethical achievement coheres with and is an inevitable consequence of saying that it does not result from Jewish ethnic identity. Paul is not always concerned with human ethical achievement, but in those texts where it does arise (e.g., Rom 4:4-5; 9:10-13; Phil 3:6) he means what he appears to say."
