From my review in the TLS:
It is fitting that Erich Hackl gives both the living and the dead, survivors and victims, voices with which to speak of the past. Presented as a series of interviews, The Wedding in Auschwitz investigates the lives of Rudi, Marga, and the many people, friends or strangers, who knew them or of them. It is a disconcerting but thematically important ellipsis that Rudi and Marga never tell their stories for themselves. Memory mixes with hearsay: the wedding, and even Rudi himself, took on an almost mythical air as news of the event spread among the prisoners, and because the facts cannot be established, this sense is passed on to the reader.
Although the record is muddied, some things emerge for certain. Rudi, a devoted Communist, travelled to Spain to fight for the Republic, and there met Marga. When the International Brigades were dissolved, Rudi escaped to France, and was interned. Hoping to be reunited with Marga, he accepted repatriation to the Reich despite the danger of almost certain political persecution, and at his insistence she followed. The decision was a bad one: Rudi was arrested and sent to Auschwitz, Marga was forced into wage slavery. It was in this demoralising, dehumanising situation that Rudi sought permission to marry, a strange act of hope at a time of deep despair.
The Wedding in Auschwitz tells an extraordinary true story, but it is the manner of its telling that makes it such an exceptional book. Some artists understand that a document can have enormous emotional power, and Erich Hackl has wisely chosen not to conceal the documentary origins of The Wedding in Auschwitz. Rather than producing the standard researched-based novel, balanced between the requirements of honouring history and abiding by the conventions of the form, he has instead given himself entirely to the subject, and embedded his own art within the record.