Mike tagged me with the question, so here are my suggestions:
I agree with Mike on the NIGTC volume by Harris and the NAC volume by Garland, as well as the monograph by Savage. One might also find Barnett helpful (NICNT) or Thrall (ICC). Another nice monograph includes Long‘s Ancient Rhetoric and Paul’s Apology (Cambridge, 2004).
another great essay is scott hafemann’s “paul’s ‘jeremiah’ ministry in reverse and the reality of the new covenant.” it’s hard to get a hold of, though, since it’s in a festschrift published in india. but if you can get it, it’s a GREAT theological piece. actually, all of hafemann’s stuff on 2 cor is good, esp. “suffering and ministry in the spirit.”
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C. K. Barrett’s 2 Cor is a bit old, but still very good — as is all his work.
Agreed.