I have been a big fan of Richard Frank's work (particularly "Guadalcanal") but have never read downfall. Next on the list!
My dad, who entered service as the war was coming to a close, made the newspapers around here back in 1995 when he delivered the Memorial Day speech in his hometown, and vigorously defended the dropping of the bomb on the 50th anniversary year. (There was a lot of that pious moral blathering going on that year, IIRC) I have always been inclined to let the validity of the decision lie in the context it was made, and I think that is enough.
David Westheimer, who wrote the novel Von Ryan's Express, wrote a novel called Lighter Than A Feather, a fictionalized treatment of the invasion of Japan.
I read it when I was younger, and was suitably impressed with the appalling aftermath of such an undertaking - particularly amongst the civilian population.
The pious liars can jam it.
http://www.amazon.com/Lighter-Than-Feather-David-Westheimer/dp/1131309553