How much did the American public know about the Holocaust?

[Spoonist]

it's not surprising that it came as a shock to the Western world when it was discovered.

There we go again. No it wasn't "discovered", nor was the shock that he nazis had death camps.
The shock came with the pictures, just like with the Ethiopian images in the eighties. All who wanted to knew that there was massive starvation in africa and Ethiopia especially. But what did shock the public into some sort of action eventually was the reporters going there filming and taking pictures. A first page picture of a bloated infant covered in flies brings home the message so much harder than a column with facts.
Same thing with the holocaust. All who wanted knew, but seeing the pictures during the nazi trial coverage really shocked people. We humans are simple that way.

[Zaune]

Further to the comments about how people weren't inclined to believe the 'civilised' Germans could commit such atrocities, if Europe and the various US foreign correspondents stationed there at least didn't have a pretty good idea how 'civilised' the Nazis were after Guernica and Kristallnacht then we sure had it brought home to us the next year.
I think it's more of a general disinclination to believe that one's fellow human beings are capable of what the Allies found when they liberated the death-camps. There was literally no precedent in all of human history for it.

[Spoonist]

There was literally no precedent in all of human history for it.

Minor nitpick here, you need to better define what you mean here else this will be false. There is plenty of precedent in history for ruthless extermination on a massive scale, or persecution of jews, etc.

[xt828]

Was there precendent for that kind and scale or organised, industrialised slaughter? I know that there were examples of mass slaughter and genocide, and examples of camp systems of remarkable cruelty, but it seems to me that the Holocaust stood apart due to the level of organisation and targeted efficiency involved in the mass murder.

[Thanas]

That depends on how you define efficient. The Roman method of just marching through Jewish towns and executing anybody on the spot was probably more efficient than the papertrail and bureaucracy-heavy Nazi machine.

That said, the Holocaust is the single most organized genocide in the modern age, so the statement is pretty true regardless, because who back then read about ancient genocides?