It's a bit pedantic, with no real plot; just a series of episodes in the life of one extremely unlucky horse. It's similar to Uncle Tom's Cabin: both were written by religious 19th century women to condemn a social ill. Both feature goodhearted, intelligent protagonists who refuse to hate or condemn their oppressors, but who suffer due to the cruelty and stupidity of their owners.Through Beauty's experiences, we see all the misery caused by poverty, alcohol, war, fashion, callousness and just plain stupidity. The human misery is intertwined with that of the animals, Sewell makes it clear that abused people tend to abuse animals, sometimes unwillingly. Lots of speeches about ignorance, alcohol, and the importance of a Sunday off.
Still there are a lot of good lessons. I like the juxtaposition of the good poor family and the good rich family; there are also stupid, mean poor people AND rich people, good children AND bad children. Sewell's point seems to be that though poor people may have a better excuse to treat their animals poorly, this is still a choice, and everyone is happier when they make an effort to be humane.