In his acclaimed book “East West Street” (2016), the British-French human rights lawyer Philippe Sands intertwines two stories that, to him, are squarely if not quite equally personal. One strand is a memoir in which he attempts to piece together the life of his maternal grandfather, a Holocaust survivor who rarely spoke about his experience or the many relatives he lost. The other is a legal history of the concepts of genocide and crimes against humanity as they were developed at the Nuremberg trials. Much of Sands’s professional life has revolved around these ideas. In 1998, he helped draft the...