I'm not an expert at all, and I haven't double-checked my memory on this one, but the folks I was thinking of were Sherwin (whose first name I'm blanking on), Kai Bird, and Barton Bernstein. Kai Bird edited an anthology on the question which was quite good, and included a number of liberal or left historians, called "Hiroshima's Shadow."
By the way, I do think people should read Alperovitz, who makes quite a good case. His Foreign Policy article is online at http://www.ncesa.org/html/hiroshima.html (apologies if someone's already posted that link).
Barry