I'll second The Aeneid
, and add my own nomination, A Clockwork Orange
by Anthony Burgess. I last read this in my late teens, and remember it as a challenging read due the future-slang employed by Burgess, a hybrid English/Russian argot. I kept having to refer to the glossary/dictionary at the back of the book to work out what was being said - difficult to start, but soon it started to flow. But be warned, this book is a real tolchock in the yarbles!
A vicious fifteen-year-old "droog" is the central character of this 1963 classic, whose stark terror was captured in Stanley Kubrick's magnificent film of the same title. In Anthony Burgess's nightmare vision of the future, where criminals take over after dark, the story is told by the central character, Alex, who talks in a brutal invented slang that brilliantly renders his and his friends' social pathology. A Clockwork Orange is a frightening fable about good and evil, and the meaning of human freedom. When the state undertakes to reform Alex—to "redeem" him—the novel asks, "At what cost?"