Because this novel is so involved with European Jewish culture, I thought it might be helpful to define a few words in the text:
1. Ba'al Shem Tov. In Hebrew, the name means something like "The Master Of The Good Name." It's the honorific title given to Rabbi Israel ben Eliezer (c. 1700 - 1760), a Ukranian mystic who is considered the founder of the strict, ulttra-orthodox sect of Hasidism, often associated with beaver fur hats and long black coats, ring curls at the ears (called "peyos" [pay-us]) and unwavering adherence to Sabbath and festival laws. Almost all the Jews in THE LAST OF THE JUST are Hasidic--as indeed were so many who were burned in the Holocaust, partly because they were so obviously recognizable on the street because of their distinctive clothing. Hasids are not only very strict but also extremely mystical, some of the most profound users of the kabbalah, a magical/mystical interpretation of Jewish scriptures. Indeed, one of the most interesting things is that the novel itself has the mystical tone of kabbalah--which gives it almost a fairy-tale or folklore quality, particularly as it moves into the German sections.
2. Levy (or Levi). According to Judaic tradition, anyone with the last name of Levy (or Levi) is a member of the ancient tribe of Levi, one of the original twelve tribes of Hebrew culture as delineated in Torah, the tribe from which the temple priests were chosen. By the way, those with the last name of Cohen are considered actual descendents of those priests ("cohen" just means "priest" in Hebrew)--and indeed, genetic studies have shown a very narrow field of variation among Cohens, a mark that they are indeed some sort of cohesive group even today.
3. Midrash. Jewish theology is actually an interlocking system of interpretive schematics. In a nutshell, Torah is the "law" (a combination of legal rules and early history) Moses received from God, the first five books of the Bible, sometimes called "the Pentateuch" (Genesis, Exodus, Leviticus, Numbers, Deuteronomy). Tanakh is what Christians call "the Old Testament," in Jewish thought itself considered a commentary on Torah. The Talmud is a commentary on Tanakh, a collection of the sayings, teachings, and interpretations of thousands of rabbis over the centuries. Many young men in Hebrew schools today study Talmud, not Tanakh or Torah. And the Midrash is a commentary on the Torah and Talmud. Thus, the Midrash is a commentary on a commentary on a commentary of Mosaic law. And thus, Jewish theology is an interlinking set of commentaries, a vast jigsaw puzzle of interpretations.
4. Mitzvah. Technically, the Hebrew word means "commandment," a reference to one of the many laws God gave Moses. However, it has come to mean something like "blessing." You do something as a mitzvah--that is, as a blessing because you're fulfilling one of God's laws. If you feed the hungry, someone might say, "Such a mitzvah!" Visiting your parents is a mitzvah.
5. Ta'anith (or taanit) This is a commentary specifically on Jewish feast days (Passover, Yom Kippur, etc.), considered part of Talmud.
6. Tsedokeh (or tzedakah). A Hebrew word meaning "righteousness" or perhaps "justice." However, in modern usage, it's now come to mean "charity"--but not what you might think. Rather, it's an obligatory charity--for example, in Jewish theology, feeding the hungry is not considered an act of volitional charity (like putting money in a church offering) but instead an act of tzedakah, an obligation, like helping your family out when they are in trouble.
And one more thing: as the novel moves into the German sections, there's much written about hats and wigs. Hasidic men must keep their heads covered at all times to show reverence to God. Indeed, most men wear a yarmulke (the skull cap sometimes also called a "kepah" [kee-pah]) as well as a more traditional black round fur hat or a black fedora. Women, by contrast, shave their heads after marriage and wear a wig (called a sheitel [shy-tul]). They do this to conform to Torah law which claims that a woman's hair is a mark of beauty--and so women should not appear attractive to other men once they have been married. Thus, women cover their hair by setting a wig over their hair--or simply shave their heads so the wigs fit better. Some rabbis also claim women must shave their heads so they will not appear too attractive and distract their husbands from studying Torah, Talmud, or other sacred texts.