Long before Luther is a book, I would suggest any Protestant to read. Nathan does an excellent job connecting the teachings of the Reformation in 1517 to the Patristic writings in the early church. This is important to understand as we are continually being accused that our Protestant beliefs are only 500 years old and are not flush with the early church or the rest of church History.
But Nathan points out the opposite. Our belief may not be kosher with the medieval church, but the jive with the Early Church Fathers beliefs and most importantly they are rooted in what the Word of God teaches. Nathan points out that our vies of Justification by faith alone, and the imputed righteousness of Christ did not begin with Luther and Calvin but was rooted in the theology of the early church. We see clearly that the Roman Catholics are the ones who have perverted the Gospel and not the Protestants. The Reformation was not an invention; it was a recovery. Nathan covers this in Chapter one that the Reformers didn't invent this; rather they recovered it. Protestant beliefs are not young; rather they are linked back to the early church.
Nathan Does an excellent Job at examining the Patristic writings and then connecting those writings with the works of the Reformation. What's nice about this work is Nathan doesn't just give you random quotes from Church history; instead, he gives you the background and setting of the quote to show you that he is not twisting what the early fathers were writing.
I would recommend this book to every Christian and Catholic to have a complete understanding of what Protestants believe/practice.