I've done some research but most books are focused upon other topics, rather than when the idea for the Holocaust was decided upon, can anyone give any suggestions?
For a general introduction to some of the debates regarding the Holocaust, some of which hit on the dating debate, you can try The Holocaust: Problems and Perspectives of Interpretation by Donald Niewyk.
If you are looking for one single perspective, try Christopher Browning's The Origins of the Final Solution. It is massive, but he is a good writer.
Ian Kershaw! Nazi Dictatorship is a good start