There were a significant number of American POWs who witnessed the bombing of Nagasaki. I don't have a concrete number, but it seems to be approximately 100 or so. There is reasonable number of Australian POWs who witnessed it, held in the same camp as the aforementioned Americans. 24 Australian POWs were in a camp approximately 2km from the middle of the blast zone, known as Fukuoka #14, and another two were in a camp some 8km away. An account of their (the Australian POWs) imprisonment is in Hugh V. Clark, Last Stop, Nagasaki (Allen and Unwin, 1985). Peter McGrath-Kerr was repairing a bridge in Nagasaki the day the bomb dropped. During a break, he and his fellow prisoners heard a single plane approaching, and quickly took shelter in a bunker. After reaching shelter, he remembers nothing for the next five days. He supposed that he was either unconscious the whole time, or the explosion gave him short term amnesia. Four POWs from his camp of approx. 200 died immediately in the blast, and about 30 were severely injured, either from the actual blast, or radiation poisoning. Four more POWs, all Dutch, died later from burns.

Perhaps the most vivid account is from Jack Johnston, another Australian POW;
"What I saw was apparently three white parachutes in triangular fashion about 60 degrees elevation. Suddenly there was a brilliant flash like a photographer's magnesium flash. Instinctively I dropped to the ground beside a kerbing at the side of the alleyway. Then came thee blast with a deafening bang and I felt as though I had been kicked in the guts. I found myself gasping for breath, pinned under a lot of rubble and unable to see... Many were injured, some with weeping flesh where the skin was blown from their bodies and hung about them like the skin of a potato that had been boiled in its jacket. During the night we looked down into the pit of fire that was Nagasaki."

Unfortunately I cannot find any American memoirs or interviews. Sometimes the available sources don't always give you what you would like.