I just found this book referenced by Tom Schreiner in Interpreting the Pauline Epistles. It is essentially a harmony of Paul's epistles. It works in the exact same way gospel harmonies work. For each paragraph of Paul's Epistles it lists corresponding paragraphs in his Epistles that address the same topics. A main strength of the book is that it is not organized by topic, but rather by Biblical book in canonical order. For example the first section is Romans 1:1-7 and then lists all Paul's other opening statements in his letters with all the text included. A Logos edition of this would be excellent!
