Over the course of the last two years, teaching Paul now four times, I have, through trial and error, found what I think are the best two Paul books for my context. I don't claim to think however that these are the most suitable for every context. What's more, while I would like to use Michael's book Introducing Paul--and do recommended it often to students, it overlaps with my own lectures (great minds think alike).
The IVP book by Capes, Reeves and Richards, Rediscovering Paul: An Introduction to His World, Letters and Theology (2007) and Horrell's An Introduction to the Study of Paul (T&T Clark Approaches to Biblical Studies) (T&T Clark, 2006). The former overviews topics like letter writing in the first century (Richard's expertise) and short chapters over all of Paul's letters. The latter is introduces students to the academic study of Paul and shows them the various approaches to his interpretation.
There is really only one other book I still have yet to find that I think would be useful as a third leg of the stool. That is a short book that provides a narrative history of Paul's life. Richard Longenecker's short book The Ministry and Message of Paul would be just the book if it were not now over 30 years old.