One of the most perplexing books of the New Testament - to me - is Paul's letter to the Galatians. In the last couple of years, as I've tried to sort out issues surrounding the New Perspective on Paul (NPP), Galatians has seemed even more puzzling than before. Two books have recently helped me.
Perspectives Old and New: The "Lutheran" Paul and His Critics", by Stephen Westerholm was first. It is a lengthy, scholarly expose of the New Perspective(s), the bulk of the book being a survey of the last 100 years of scholarship on Paul. Frankly, much of it was dry reading (at least for me - which is one reason it took me so long to read). However, the first part of Westerholm's book is a very helpful overview of the Pauline soteriologies of Augustine, Luther, Calvin, and Wesley (all "Old Perspective" in their understanding of justification). And, most helpful of all, in the last part of his book, Westerholm develops his own understanding of Paul's theology through a careful exegetical study of righteousness, the law, and justification in Paul's letters. Those last 150 pages or so were immensely helpful, and started clearing the cobwebs from my brain.
The second book, which I read as a follow-up to Westerholm, is John Stott's The Message of Galatians: Only One Way. Stott's book is a very helpful expositional study of the epistle, from a traditional Evangelical/Reformed perspective. It doesn't interact with the New Perspective (it was written before NPP took flight), but rather a simple, to-the-point unfolding of Galatians. It was very good and helped me even more.
I suspect the fruit of these two books will show up in future writing and preaching, but for the meanwhile, if you are having trouble with the NPP, get Westerholm. And if you are puzzled by Galatians, get Stott.