The question of why and how Hashem could allow the Holocaust to happen is an unanswerable question if you maintain the premises that Judaisim does about Hashem (that he is good, just and fair etc.) and for the most part and for most people books like the one mentioned by the OP raise more questions then they answer.
This does NOT mean that an answer does not exist. It just means that if one does exist it is fundamentally beyond our capability to understand it (the same way it is impossible for a monkey to grasp the concepts required to do calculus or algebra etc)