•Why it earns a slot
Why it earns a slot: it built the modern novel out of one running joke, a man's private delusions colliding with a world that will not accommodate them, and the joke coined its own idiom, "tilting at windmills," that outlived every knight-errant story it was mocking.
A minor nobleman reads himself insane on chivalry romances, then rides out as a knight-errant to fight giants that turn out to be windmills.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.