"1984, Animal Farm and Lord of the Rings"
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Fantastic choices, but I have found with today's youth you must explain JRR Tolkein's analogy in his fiction with actual history. They are so overwhelmed by the fantasy, they do not correlate it to actual events.
The horrors of the 20th Century resulted from two things -- centralized authority and the moral corruption that results when human beings exercise ultimate power over others. 1984 and Lord of the Rings address these issues more powerfully than other books written during the last 100 years -- which is why they are the two most important books written during the 20th Century.