The link provided will give you direct access to these quotes, plus many more that could be beneficial to your work. For your reference, all my writings are in bold, while the direct quotes are in regular type. When Truman took over he had to play catch up, and was very behind in terms of the intimate strategy of the war.
3/3/48 Letter to Margaret Truman: "As you know I was Vice-President from Jan. 20 to April 12, 1945. I was at Cabinet meetings and saw Roosevelt once or twice in those months. But he never did talk to me confidentially about the war, or about foreign affairs or what he had in mind for the peace after the war... Then I had to start in reading memorandums, briefs, and volumes of correspondence on the World situation. Too bad I hadn't been on the Foreign Affairs Committee or that F.D.R. hadn't informed me on the situation."
I believe Truman was excited about the bomb, because it gave him a clear path to victory. His writings bear that he had no love, nor hate, for the Japanese people, but he was intent on ending the war and dealing with his enemies accordingly. He was amazed by the destructive power of this new weapon, and thankful that it wasn't first developed by our enemies. He sees the weapon as a strategic means to end the war.
7/25/45 Diary Entry: "We have discovered the most terrible bomb in the history of the world. It may be the fire destruction prophesied in the Euphrates Valley Era, after Noah and his fabulous Ark... Anyway we 'think' we have found the way to cause a disintegration of the atom. An experiment in the New Mexican desert was startling - to put it mildly. Thirteen pounds of the explosive caused the complete disintegration of a steel tower 60 feet high, created a crater 6 feet deep and 1,200 feet in diameter, knocked over a steel tower 1/2 mile away and knocked men down 10,000 yards away. The explosion was visible for more than 200 miles and audible for 40 miles and more...The weapon is to be used against Japan between now and August 10th. I have told the Sec. of War, Mr. Stimson, to use it so that military objectives and soldiers and sailors are the target and not women and children. Even if the Japs are savages, ruthless, merciless and fanatic, we as the leader of the world for the common welfare cannot drop this terrible bomb on the old capital or the new [Kyoto or Tokyo].
"He [Stimson] and I are in accord. The target will be a purely military one and we will issue a warning statement [known as the Potsdam Proclamation] asking the Japs to surrender and save lives... It seems to be the most terrible thing ever discovered, but it can be made the most useful."
Truman didn't romanticize about the bomb, but he felt it was a necessity and was willing to use it to save American lives and end the war. He believed he was justified by giving the Japanese fair warning and was dismayed by their "pigheadedness" in forcing his hand.
8/11/45 : "Nobody is more disturbed over the use of Atomic bombs than I am but I was greatly disturbed over the unwarranted attack by the Japanese on Pearl Harbor and their murder of our prisoners of war. The only language they seem to understand is the one we have been using to bombard them. When you have to deal with a beast you have to treat him as a beast. It is most regrettable but nevertheless true."