The most interesting, and famous, ending in the book is the conclusion of "Bartleby". At the conclusion of the story, the narrator reveals that Bartleby may have been a clerk in the dead letter office—a department that deals with undeliverable mail. The narrator imagines that Bartleby may have been depressed by all the letters that could not be delivered, and so the narrator ends by declaring sadly, "Ah, Bartleby! Ah, humanity!"
The sympathy could be read as sympathizing with everyone who confronts the futility of life, where you life and die and your letters go to some bureaucrat who finds them depressing. But it could also be a less universal sadness, not about where humanity has always been, but about where it's going, or isn't.
The dead letter office in the post office was only established in 1825; 28 years before Melville published "Bartleby." Rather than a timeless human truth, a centralized bureaucratic post office was a relatively newfangled thing, part of an expanding white-collar economy filled with more clerks and more office jobs and more mid-level grunts like Bartleby. The end of the story, then, is in part about the way in which Bartleby and humanity are becoming one and the same; in which people are becoming their paper trails, and dead letters are merging with dead people. Bartleby's fate is all humanity's fate not because everyone dies, but because everyone is turning into a bored bureaucrat. No wonder the narrator sighs.