I know that. And I said why: to save face. He didn't want to tell the Japanese people that the Soviet invasion of Manchuria was the reason. The atomic bomb made a much better reason for public consumption. That doesn't mean it was the actual reason that drove the decision, which was made in private.
That's not clear either. Without the atomic bomb they might well have found another way to save face.
Now I will say that I have not read Hasegawa's book but Wikipedia says that it is "challenging the widely accepted orthodox view that the atomic bombings on Hiroshima and Nagasaki were the most decisive factor in Japan's decision to surrender ending the war against Japan."
It will be interesting to read but it looks like that it is not the accepted theory and that primary sources from the era were destroyed on all sides.
The emperor directly mentions the atomic bomb in his address; "Moreover, the enemy has begun to employ a new and most cruel bomb, the power of which to do damage is, indeed, incalculable, taking the toll of many innocent lives. Should we continue to fight, not only would it result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization."
He only tacitly mentions the battle situation; "But now the war has lasted for nearly four years. Despite the best that has been done by everyone – the gallant fighting of the military and naval forces, the diligence and assiduity of our servants of the state, and the devoted service of our one hundred million people – the war situation has developed not necessarily to Japan's advantage, while the general trends of the world have all turned against her interest."