A lot of the Holocaust denial comes from racism thickness mentalhealthissues some guy in the 1960s, I think it was, who was an industrial engineer who insisted that he could prove that the gassing and chimney system at Auschwitz as described and depicted in photographs wouldn't work, and thus people were lying and had falsified pictures. This idea became quite popular at the time,even though it was quickly proved that he was totally wrong, and was a massive far-right anti-semite.
As Wilson points out, Europe was so incredibly anti-semitic at that time that it's hard for our modern minds to grasp (and there's still a huge problem with it, even in the UK). Most people know stuff about Oswald Mosley, and how the Daily Mail backed Hitler and the nazis; lesser known is that the Queen Mother was a supporter of nazi policies, and that it was said that Edward VIII was actually forced into abdication not entirely because of his affair with Wallace Simpson, but because he was a close personal friend of Hitler. Also that the UK, along with the rest of the world, was given the chance to take Jewish refugees in the 1930s, at a global conference (date and place escapes my mind) where Germany was totally explicit about what they were going to do to Jews, and every country except for a couple of small ones, like Dominique and the Netherlands, refused.
People also always say that no-one knew what was going on with the Jews and other persecuted groups under the nazis, and yet, every old person I've ever talked to on the subject always says that most people knew, and it was in the papers, and on the radio, at the time, and that people just wanted to pretend they didn't know, then and now.
The best book on the topic that I've read, is The Holocaust by Martin Gilbert. It is totally comprehensive, and he is a world-reknown expert on the topic. Another good, but chilling book is Hitler's Willing Executioners by Daniel Goldhagen, which exposes just how much the German population (and others) were involved in the murder of the Jews and other groups. I will never forget reading about how ordinary men used to beg to be allowed to take part in the shootings of Jews into mass graves, and that the things they did to them, before and after death, was so abhorrent that the SS soldiers who undertook the shootings on a regular basis were traumatised by it.