•Why it earns a slot
The founding text of the free-press argument in the English language, written 350 years before anyone used the phrase 'marketplace of ideas,' and the direct ancestor of First Amendment reasoning.
Milton wrote Areopagitica as an unlicensed pamphlet attacking Parliament's own 1643 Licensing Order, which required government approval before anything could be printed. He argues that suppressing a book is close to killing a rational being, that virtue tested by nothing is not virtue at all, and that truth wins any open fight with falsehood, so censorship protects error more than it protects the public.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.