Alfred Döblin was of Jewish origin himself, but his background was so secular that he couldn't really relate to his Jewishness. After some first pogroms in 1920s Germany, Döblin decides to travel to Poland to find out about the traditional Jewish way of life. Döblin is shocked. Life in the ghettos is horrible, the places are overcrowded and dirty. Most Jews lead a life completely apart from the Polish people around them. They didn't have the chance to lead a "normal" life, as they were discriminated against in a way which seems to be an early form of apartheid. This way of life must influence people's view of the world: Döblin is struck by the religious fanaticism, which he finds fascinating and repulsive at the same time.
Of course the book tells you a lot about the short-lived Polish Republic of the 1920s too, but the Jews are Döblin's main interest. He describes a civilization which was completely destroyed by the Nazis only a few years later. Of course Döblin's view is that of an outsider, but he is a highly civilzed and sympathetic outsider, and I am sure his picture is a lot more realistic than all those of idyllic Schtetels where bearded people are busy being merry and making music.