Roland Bainton’s
Here I Stand is the most readable treatment; Heiko Oberman’s
Luther: Man Between God and the Devil is the best (in my opinion). David Bagchi’s
Luther’s Earliest Opponents is a helpful study of the earliest Catholic responses to Luther (from a Protestant perspective; I don’t agree with all Bagchi’s interpretations and you would be even less likely to, but he raises interesting points, claiming for instance that Catholics had trouble explaining just how Luther’s teaching about faith was heretical).
In terms of primary sources, there are three early texts that set forth Luther’s program quite clearly:
Freedom of a Christian,
Babylonian Captivity, and
Appeal to the German Nobility. The 1535
commentary on Galatians is his fullest discussion of his doctrine of salvation. The
Large and Small Catechisms are handy summaries of basic Christian doctrine as Luther understood it (try the Small Catechism–it is, obviously, smaller!).
The Augsburg Confession (written largely by Melanchthon–it’s not Luther’s work) is and remains the most official statement of Lutheran doctrine. The 1537 Schmalkaldic Articles, which are by Luther, are more radical and are also authoritative for Lutherans. Both of these texts, along with the Catechisms, can be found
here. The same website also contains the Confutatio Pontifica–the Catholic refutation of the Augsburg Confession.
John Eck, one of Luther’s earliest opponents, wrote a book trying to refute Protestantism systematically by arguing from Scripture. This “Enchiridion” is available (in good academic libraries) in a very bad translation by Ford Lewis Battles. Or you can read the Latin text in the series “Corpus Catholicorum.”
The definitive Catholic response to Protestantism in general is, of course, the Council of Trent. Martin Chemniz, the greatest Lutheran theologian of the later 16th century, wrote a lengthy refutation called
Examination of the Council of Trent which has been published in English in four volumes.
Edwin