The bulk of the argument is in the 3rd chapter where Piper presents exegetical arguments for imputed righteousness, particularly in Romans (and other related pauline texts). Although, the rest of the book – especially the first 3 chapters – could appear to be padding in order to convert an essay into a book (which it is); it does also provide the pastoral and practical background to the arguments presented against Gundry. This sets Piper apart from his opponents in the same way Augustine differed from Pelagius – one was coming from the hard slug of pastoral concerns, the other mostly academic.
Chapter 3 could either be a hard slug or the meat of the matter depending on who is reading. Expect references to the Greek text. The general advice to read just the Introduction and Conclusion (chapter 1,2 and 4) to get the gist of the matter applies here also. Wade into chapter 3 at your own peril.