Gulliver's Travels by Jonathon Swift
I don’t think I read this as a kid, or if I did I only read the Lilliput part, though I did remember that Gulliver also met big people who treated him like a little doll. Those were parts 1 and 2. In part 3, he visits a number of different places trying to get home. He encounters people whose dedication to science makes them incapable of doing anything practical and some humans who never die but continue to age, being “written off” by their culture when their contemporaries die so that the live a painful and miserable death. Finally, in part 4, he lands in the land of the Houyhnhnms who are rational horses who don’t even have words for lie, deceit, murder, etc. There are also Yahoos, and they look just like Gulliver but are assumed to be completely irrational because all they do is scrap and fight. Gulliver is finally forced to leave because he appears to be a Yahoo—even though he’s made friends among the Houyhnnms. He makes his way back to England and turns in revulsion from his wife and children and all other humans, being so traumatized by the Yahoos and his own sense that he’s really one of them.
The satire is funny, even when you don’t track down all the specific references to Swift’s contemporary world, but the last part where Gulliver is revolted by the Yahoos and their behavior is no longer so funny. The reader is tempted, like Gulliver, to revile one’s own kind who, even 300 years later, are still busy lying, cheating and making war.