Digital books
In Fahrenheit 451, books were preserved by people memorizing them.
What's really cool is that nowadays you can carry an entire library on a chip small enough to hide under your tongue. The hard part in a "451-type" world would be hiding the devices to read them. Right now the smallest ones are about the size of a pack of cigarettes, with the limit of course being a screen big enough for a human eye to pick out words.