The story by Heinrich Mann (brother of the more famous Thomas) dates from 1905. It is known to non-German-speaking people mainly through its famous film adaptation from 1930, The Blue Angel, with Marlene Dietrich and Emil Jannings.
In the novel, Professor Raat is a stuffy, straitlaced high school teacher who delights in disciplining and correcting his students in an angry, demeaning way. You might wish to compare him with similar old-fashioned teachers in literature who enjoy insulting and humiliating kids in class, such as Mr. Crocker-Harris in Terence Rattigan's The Browning Version. The students hate Professor Raat , and one of them adds the prefix "Un" to Raat's name written on the cover of a copybook. In German, "Raat" and "Rat" are pronounced identically, and the word Unrat means filth, garbage.
Raat becomes obsessed with a beautiful cabaret dancer named Rosa, who is probably a prostitute. The obsession ends up being Raat's undoing, and though he marries Rosa, it's clear that she is seeing other men. Raat has lost his respectable teaching job, has blown all his money on Rosa, and has basically destroyed his own life.
You can look into the question of whether these plot elements were already common in European literature in the early twentieth century, or whether this was a seminal work of fiction that perhaps created what became clichés. Does Mann see Raat's obsession with Rosa as purely destructive, or is it liberating to him as well, freeing him from the constrictions of a false and repressed life? Does he actually become a better or happier person when he links his fate to her, or is he throwing away a valuable life and simply making a fool of himself? These questions and issues form the heart of the novel's theme.