While it does not deal with "SS concentration camp guards" but rather reserve police, Browning's Ordinary Men is by far the best work for insight into this that I know of. It is based off a case study of a reserve police batallion tasked with the destruction of Jewish communities in Poland, including summary executions and rounding victims up to be sent to extermination camps.

Be warned that it is a very gruesome read; I found it quite emotionally taxing. It cites some very vivid testimony from the men involved.