How has there come to be a Jewish population more than 100,000 strong in the land Hitler promised to make "Judenrein"? As Gay shows in this intriguing if uneven history, Germany ironically served as a haven for Eastern European Jewish immigrants immediately after the war, when a quarter-million Jews were housed, under the protection of the Allies, in displaced persons camps. Gay's discussion of this is the strongest part of her book, as she deftly examines the dynamics of the "surviving remnant" of European Jewry as it tried to rebuild itself amid the ashes of the Holocaust, creating schools, arts organizations and families. As Gay notes poignantly, "the last flowering, the last living moment of Polish Jewish culture, played itself out in the D.P. camps in Germany." As soon as they could, however, most of the Jews emigrated. Germany's Jewish population then struggled with small numbers and the legacy of Nazism for more than 40 years. As Gay (who won a National Jewish Book Award for Unfinished People: Eastern European Jews Encounter America) tracks the life of Jews in both East and West Germany, the book loses focus. But as she details the influx of tens of thousands of Jews from the former Soviet Union in the 1990s, the narrative picks up again as she shows how this latest set of refugees has the opportunity to create a new, vibrant German Jewry. 30 illus. (Sept.)