I'm a bit late here, but this is a subject that interests me and I have read a bit on it. To me, the evidence for a mass slaughter in the Vernichtungslager is undisputable. A very good book on the deniers is "Denying history" by Shermer and Grobman (Univ. of California 2002) - my copy is sitting beside me here on the bookshelf. I became interested in the business when I encountered a Holocaust denier on another website. He tried to quote chemical evidence to show that gassing with Zyklon B couldn't have taken place (the celebrated "staining" argument), and I (honours in chemistry who had worked for a company skilled in cyanide chemistry) and an entomologist (skilled in the use of HCN as a pest exterminator), tore him into little shreds.

The best answer to Holocaust denial is the father of a former secretary of mine, He stood in that queue in Auschwitz with that SS guard pointing. He, a young, fit man, went that way, the rest of his family (nine of them) went that way. He has the great good fortune to be Hungarian, and the Nazis came for the Hungarian Jews late in the war (1944). He survived just under a year of hard labour before the Russians liberated the camp.

Many years later in America (to which he'd emigrated), someone wrote in the local Pittsburgh paper that it hadn't really happened. For the first and only time, he wrote a letter to a newspaper. He was delighted, he wrote, to know that his family really hadn't gone up the chimney at Auschwitz, and could the writer please inform him where he could see them. Strangely, no answer was forthcoming.

Another good read, now back in print, is Sir Martin Gilbert's  "Auschwitz and the Allies".