I hardly know why I 'really liked it' - for some reason it kept me on the edge of my seat. And that even though it has a number of structural flaws as well as a less-than-clear message (although that might have been part of the fascination).
The title "Professor Unrat, or the End of a Tyrant" makes you expect the Professor to be the bad guy but (at least for my taste) everybody in this book is equally bad, with the exception, perhaps, of the actress Fröhlich, who only has loose morals but no ill-will. Unrat is a misanthrope because he has never had any positive experiences with other humans, and he suffers from a dangerous mix of inferiority complex and paranoia. Everybody else in the small town knows he's the one to hen-peck - and the more his insecurity tickles people's urge to peck, the more this feeds his paranoia and re-inforces his insecurity. He is, however, a high-school teacher - the wrongest job he could have possibly had - and as such believes it his right to rule every aspect of his students' lives - which his position in town as well as his complexes of course prevent him from doing other than in his imagination. (The name is an insult derived from his actual name: in translation you might say his real name was 'Tash' and everyone calls him 'Trash' - students and citizens alike say things like 'smells like trash' when he goes by.)
When he gets involved with Rosa Fröhlich, it's the first time he idolises another person - and she, in addition to being inclined to encourage anyone with money, is flattered enough by his idolising to develop a certain real liking for him. They get married and after setting up a house find themselves becoming the centre of attention of the whole town. All males vie for Rosa's attention, guests come to gamble, and Unrat finally finds a sense of power in the ability of his house and wife to ruin the fortunes and reputations of those who used to call him names.
However, when the one person he sees as his main enemy comes near his wife, he loses it, attacks both wife and intruder, who calls the police. By the time they arrive, an angry mob has already gathered, happy to help bring down one they now can despise again.
(And if this summary doesn't remind you at all of "The Blue Angel" - the film made from this book - I don't blame you. The two didn't seem to have much in common, from what I could remember about the film.)
The book is quite short, and the final resolution is brought about a little too quickly and simply, without much motivation or plot development. There is also a lot of explicit interpretive commentary not only from the narrator, but, esp at the end, even from characters. These make the narrative a little awkward sometimes. However, it is an early novel, so I suppose Mann was still learning at the time. The book, first published in 1905, also uses a lot of now obselete vocabulary, and although I was mostly ok with it, I think a modern edition would do well to add a glossary.
This edition has a long critical essay at the end. From a quick read through it, it seems that Mann's own politics at the time of writing were just changing from ultra-conservative to liberal/progressive, which might explain why neither the proponents of the old order nor those of a new come off well here.