Finally, it is important to remember that no war has ever been brought to an end simply by indiscriminate bombing and mass killing of civilians. Indeed, there is abundant evidence that such strategies typically strengthened resistance.
http://www.fpif.org/fpiftxt/650
Not exactly the strongest soft source, but it's a start.
Another vital lesson - one that has taken even air specialists by surprise - relates to the behaviour of civilian populations under air punishment. It had generally been assumed that aerial bombardment would quickly shatter popular morale, causing deep civilian reactions. ... The progress of this war has tended to indicate that this expectation was unfounded. ... These facts are significant beyond their psychological interest. They mean that haphazard destruction of cities - sheer blows at morale - are costly and wasteful in relation to the tactical results achieved. Attacks will increasingly be concentrated on military rather than on random human targets. Unplanned vandalism from the air must give way, more and more, to planned, predetermined destruction. More than ever the principal objectives will be critical aggregates of electric power, aviation industries, dock facilities, essential public utilities and the like." -- Major Alexander Seversky, US Army Air Force, Victory Through Air Power (NY, 1942).
After the war, the German Minister for Armaments Production, Albert Speer, professed shock that "vast but pointless area bombing" was being continued in favor of highly effective precision bombing.[10] According to Speer, the failure to continue regularly attacking Schweinfurt allowed the Reich to escape a "further catastrophic blow" because "armaments production would have been crucially weakened after two months and after four months would have been brought completely to a standstill."[11] "What really saved us," Speer continued, "was the fact that from this time on the enemy to our astonishment once again ceased his attacks on the ball-bearing industry."
http://www.worldfuturefund.org/wffmaster/Reading/war.crimes/World.war.2/Bombing.htm#AMERICAN OPPOSITION TO AREA BOMBING
In the case of the air war against the Third Reich, bombing only caused serious economic disruptions in the final year of the war, roughly from June 1944 to May 1945. By this time a German military defeat was pretty much a foregone conclusion. Based on such results, it is impossible to demonstrate that the bombing campaign would have achieved an economic breakdown of Germany since by the time such destruction was being caused, the fronts were already collapsing in both east and west. The Soviet Union, for all intents, had won the land war by the middle of 1944 and the successful Normandy invasion delivered the coup de grace. To make a case for the bombing campaign being decisive the reverse would have to be expected. That is, the fronts would have had to collapse after the industrial damage was done.
http://www.onwar.com/articles/9809.htm