"All survivor literature talks about this numbness"

Numbness is a common characteristic of extreme psychological trauma.  The painful emotions associated with deep suffering can be too overwhelming for the psyche to process, which leads to an inability to feel even positive emotion.  The individual can therefore appear very detached, cold and distant from the people around them.  It is often related to PTSD, demonstrated by the ‘thousand yard stare’ of people who have suffered during war time.  Many well known texts by survivors of the camps are characterised by this same sense of numbness in the writing.  Most of the authors who began to write of their experiences soon after the war, such as Levi and Borowski, are notable for the way they relied on factual description, with the emotional pain, suffering and fear implied between the lines.  There is an interesting blog about Holocaust related trauma here.