A friend asked for some recommended titles to read on Paul. That's a challenge as the number of books written about Paul and his Epistles are the largest collection of books on the Bible in any library. In an attempt to provide some titles, I’ve divided the books into sections ranging from very introductory material to more specific works.
The following give a good introduction for a lay person, and introduce a reader to the issues that are to be considered. Although largely written in the 90’s, these are still available on line at Amazon as one potential source. Sometimes they have a later imprint date than the version listed.
Gager, John G. Reinventing Paul. Oxford: Oxford University Press, 2000.
Horrell, David G. An Introduction to the Study of Paul, New York; T&T Clark, 2006
Sanders, E. P. Paul. Oxford, New York: Oxford University Press, 1991.
Wenham, David. Paul and Jesus; The True Story. Grand Rapids: William B. Eerdmans, 2002.
Wright, N. Tom. What Saint Paul Said?: Was Paul of Tarsus the Real Founder of Christianity?Cincinnati, Ohio: Forward Movement Publications, 1997.
Young, Brad H. Paul the Jewish Theologian: A Pharisee Among Christians, Jews and Gentiles.Peabody, Massachusetts 01961-3473: Hendrickson Publishers, 1997.
To address some of the more specific areas of Paul’s writings, my suggestions include the following. Although one commentary is listed here, it is more for what is contained in the Appendix to the book, rather than the commentary itself.
Bassler, Jouette, Navigating Paul: An Introduction to Key Theological Concepts, Louisville:Westminster John Knox Press, 2007.
Esler, Philip F. Conflict and Identity in Romans: The Social Setting of Paul's Letter.Minneapolis: Fortress Press, 2003
Hengel, Martin. The Pre-Christian Paul. Philadelphia: Trinity Press International, Philadelphiaand SCM Press, London, 1991.
Sanders,E. P. Paul, the Law and the Jewish People. Minneapolis: Fortress Press, 1983.
Wenham, David. Paul, Founder of Christianity or Follower of Christ. Minneapolis: Wm B. Eerdmans, 1995.
Ziesler, J. A. Pauline Christianity. Oxford, New York: Oxford University Press, 1983.
For more advanced study purposes, then I would recommend the following:
Bird, Michael. The Saving Righteousness of God: Studies on Paul, Justification and the New Perspective. Edited by Howard I Marshall, Richard J. Bauckham, Craig Blomberg, Robert P Gordon and Temper Longman III. Paternoster Biblical Monographs. Milton Keynes: Paternoster Press, 2007.
Hafemann, Scott J. Paul, Moses and the History of Israel: The Letter/Spirit Contrast and the Argument from Scripture in 2 Corinthians 3. Edited by Howard I Marshall, Richard J. Bauckham, Craig Blomberg, Robert P Gordon and Temper Longman III. Paternoster Biblical Monographs. Milton Keynes: Paternoster Press, 2005.
Tomson, Peter J. Paul and the Jewish Law: Halakha in the Letters of the Apostle to the Gentiles. Van Gorcum, Assen, Netherlands: Fortress Press, Minneapolis, 1990.
Wright, N. T. The Climax of the Covenant. Minneapolis: Fortress Press, 1991.
Ziesler, John. Righteousness in the Writings of Paul. Chico, Ga.: SBL, 1978.
The Tomson and Ziesler books would probably only be found in an academic library, but if accessible, are well worth referencing.
Lastly the following individuals writing on Paul has only been in journals. However Pamela Eisenbaum, as a Jewess, brings a very interesting and useful perspective to Pauline studies.
Eisenbaum, Pamela. "A Remedy for Having Been Born of Woman: Jesus, Gentiles, and Genealogy in Romans." Journal of Biblical Literature 123, no. 4 (2004): 671-702.
And finally, anything by Martin Hengel, or E P Sanders, is worth a read. They, perhaps more than others have done much of the foundational work in this area.