Glavlit and samizdat
The state pre-cleared everything printed; readers answered with carbon copies.
What happened
Glavlit, founded in 1922, pre-cleared everything printed in the Soviet Union — books, newspapers, labels, even registering the typewriters that could reproduce text. Readers answered with samizdat ("self-published"): retyped carbon copies passed hand to hand, and tamizdat, works smuggled abroad to be printed and smuggled back. From 1968 the samizdat Chronicle of Current Events catalogued arrests and trials for fourteen years. All of it was a criminal act that could carry a camp sentence.
The results
A parallel, illegal literature kept dissenting thought alive at real personal risk. Václav Havel's insight (The Power of the Powerless, 1978) was that the system ran on public assent to things nobody actually believed — so simply declining to repeat the lie, living "as if" one were free, counted as a political act the state could not tolerate.
Why it happened
Total censorship was never about any single banned book. It was about maintaining the fiction of unanimity on which the regime's legitimacy rested. Control of the printed word is control of what can be thought together in public — and a one-party state cannot afford a public it does not script.