From the preface to the 2nd edition (p. ix, line breaks added):
It is tempting to enlarge the book significantly, but I believe the book has continued to be read because of its brevity. Hence, the purpose of the revision is to update the book where necessary, especially in terms of bibliography.
The book has not changed dramatically, for I am still convinced that the substance of what I wrote some twenty years ago is correct. Nevertheless, the entire book has been revised, and there are some significant additions.
The original edition presented the diagrams in Greek but not in English, and thus English has been added to enable readers to understand diagramming conventions.
The most valuable chapter in this book—or at least the one that most strongly influenced me—is “Tracing the Argument” (pp. 97–124). It revolutionized how I read Paul.
Update: That chapter is available as a PDF (though it’s from the first edition, not the second).