<em>Apostle of the Crucified Lord: A Theological Introduction to Paul and His Letters</em> by leading Pauline scholar Michael J. Gorman is really helpful and happens to be only $5.99 on the Kindle at the moment. He’s not Reformed, but his emphasis on participation in Christ is very union-with-Christ adjacent. After 160 pages of introductory material on Paul’s world, theology, and spiritually, he has a substantial chapter on each of Paul’s epistles (70 pages on 1 Corinthians).