A novella is shorter than a novel, but longer than a short story. I recently read three classics:
Hemingway's The Old Man and the Sea helped push him over the top in winning a Nobel Prize in Literature. The mystical tone of the story is somewhat reminiscent of the short story "The Big Two-Hearted River", which is a good thing.
In Candide, Voltaire savagely indicted the notion of "philosophical optimism" - that everything happens for a good purpose and that "this is the best of all possible worlds." After enduring much violence and misuse, Candide and his companions finally find a reasonable existence in performing useful work as a household community. In the last line, Candide says that "we must cultivate our garden."
Wilde's The Picture of Dorian Gray is filled with unappetizing characters. It's a good study of how not to live one's life.