Victor Klemperer's diaries of the Nazi years are an important historical source because they are the only known eyewitness account by a Jew of the entire span of the Nazi years from 1933–1945. However, although they are important primary-source documents, offering an enormous wealth of detail about what life was like for a Jew in the Third Reich, the caveat is that they are the subjective account of one person. They are not a rigorous or objective historical analysis of the period. They should be used in tandem with such secondary sources.
Klemperer, a middle-class man married to an "Aryan" gentile woman, is not representative of the Jewish experience as a whole. First, he was an assimilated Jew, meaning he had converted to Christianity and was a member of the Confessing Church, a breakaway from the official Deutsch Church that barred people of Jewish ancestry. (It is important to note that in Nazi Germany, Judaism was a racial, not a religious category: a Jew was still counted as Jewish even if they converted to a different religion.) Further, Klemperer had an unusual degree of shielding due to his Aryan wife and, as college professor before his ethnicity barred him teaching, an analytical ability not available to most people.
Nevertheless, his diaries document as it happened the increasingly harsh situation even assimilated Jews in Nazi Germany faced, especially after the start of World War II. Further, the diaries are immensely valuable in that Klemperer recorded his emotional and psychological responses to the regime, including his anger and intense anxiety, as well as carefully documenting such experiences as being jailed, observing the nature of Nazi propaganda, being forced into a crowded Jewish boarding house, and fleeing Dresden with his wife during the US firebombing of the city. Klemperer provides an invaluable, incomparable source for anyone looking to understand the day-to-day "feel" of life as a precarious Jewish living under a totalitarian regime.