I've just finished reading Primo Levi's book about his time in Auschwitz.
He wrote as he said "to be a witness, not a judge", and even while in the camp made fugitive jottings on scraps of paper, although all had to be destroyed; had they been found he would have been executed.
He survived and was repatriated after a long and winding railway journey lasting nine months (the subject of a second book). Once home he soon started work on If This is a Man and had it finished by the end of 1946, less than two years after he had been freed from the camp.
Levi believed in rationality and wrote in a very objective way in order not to dilute his testimony. He hadn't been very good at Italian at school — more inclined to science — but the urge to tell his story to the world made him into a great writer.