Perspectives from the time:
Brunhilde Pomsel
Joseph Gobbels 105-year old former secretary claims she knew nothing. Link to Article (there is a documentary film as well).
Jakob W
"I do not feel like a criminal." A former guard at Auschwitz is interviewed about his experiences.
Alexander Huryn
"I was told, take the job or die". A former labour camp guard speaks.
Modern Perspectives:
Daniel Goldhagan and opponents
Daniel Goldhagan, author of the famous book Hitler's Willing Executioners, debates the question with two other scholars in this article (PDF).
This video summarises Goldhagan's ideas:
Christopher Browning's 2004 book The Origins of the Final Solution argues that the Germans who participated in the Holocaust were forced and brainwashed into doing so. Here is a PDF of that whole book! My suggestion is to start with the last section, entitled "The Germans and the Final Solution".
Robert Gellately
Robert Gellately is another who believes that the Holocaust was widely known about. His book is Backing Hitler: Coercion and Consent in Nazi Germany.