Are you focusing on their perception in the US, Asia, in Japan itself, or all the NATO states?

If you try to discuss all of them, you'll probably end up with a 300+ page essay, because the issue of Japanese war crimes is incredibly messy, given the sheer number of nations involved in covering every atrocity.

I will say this though, the "cover up" wasn't so much policy as it was practicality: the US was mostly concerned with the treatment of US prisoners at the time, with the expectation that other countries would follow up for their own people with documentation. This being the era of paper, there wasn't much of an exchange of information between the countries, and usually amounted to "Japanese soldiers shot a bunch of civilians in this area" anyway.

Attaching actual names to such events was basically impossible without resorting to torturing soldiers for information ("Itou killed someone? Which Itou? You don't know? What a pity.") and killing thousands of Japanese soldiers just in case would somewhat defy the point of trying to rebuild Japan.

Regardless, I do recommend starting with "Japanese War Crimes Records at the National Archives," because it's both free, and goes into a lot of the difficulties with the war crime trials. For the Japanese cultural shift itself, you should probably decide on a specific time and place to first.