Which letters did Paul write before Romans?
Paul wrote Thessalonians before Romans. Galatians was also written before Romans. The context indicates that other letters were written before Romans, and Corinthians may have been written before Romans as well, though with some uncertainty. So the letters before Romans include Thessalonians, Galatians, and likely Corinthians.
Les teaches that while Paul's epistles always appear in the same order in every New Testament manuscript—Romans, Corinthians, Galatians, Ephesians, Philippians, Colossians, and so forth—that is not the order in which they were written. Romans was not the first letter. Thessalonians were written first, and then even other letters were written before Romans. Galatians was written before Romans, and Les thinks even Corinthians was written before Romans. So the letters Paul wrote before Romans include Thessalonians and Galatians, with Corinthians as a likely additional letter before Romans, based on Les's recollection.