Junia, A Woman, An Apostle by David Williams is a thorough examination of Romans 16:7. The book is intended to introduce general readership to the technical arguments for the conclusion that the person spoken of in this verse was a woman apostle.
Craig Keener's 1-2 Corinthians is a wonderfully engaging and easily read commentary on Paul's letters to the Corinthians. It is tightly packed with documented information from ancient sources on the historical/social/cultural setting of Corinth in Paul's time. This information enables the reader to understand more clearly the intentions behind Paul's letters to the Corinthians, underlining how the cultural emphasis on rhetoric in Paul's time shaped his writings.
Alan Johnson's work on 1 Corinthians is particularly engaging. His reference notes and bibliography provide an entry into further study if desired, all while maintaining an appealing readable style. He deftly bridges the two horizons of the Greco-Roman culture and American culture.