My theory about the book people
Okay... the book people, or however they are called in English. No. That just doesn't work out as presented in the film. I'm not sure if the movie has just massively butchered the book or the book started it, but clearly the idea that the book people help preserve knowledge for future generations is absurd as presented in the movie, too jarring for suspension of disbelief to apply. However, I've thought about the issue and I now have a theory that helps explain the "book people phenomenon". (Bear in mind that this may or may not apply to the book, only for the movie.)
Let's just look at the facts.
*Why are the book people not persecuted? This is a dictatorship, it's not going to play fair just because, technically, by the letter of the law, they do not own books. The colony would get shut down the moment a bureaucrat feels like dealing with it. This is suspicious.
*The overall activity of the book people does MORE to destroy knowledge than to preserve it, actually. So they memorize books... even short poems get misquoted and mutated over time because of the tricks our brain plays on us. The people themselves will eventually start forgetting parts etc. And the worst freaking thing? They burn the freaking books once they've memorized them! Why? That means they are now the single point of failure! If they die ahead of schedule, or suffer brain damage, or just want to give up the book-person life and readjust into normal society, potentially an entire book is lost! What if the old man in the end dies before he could teach his kid the whole book? If the point is just to avoid being caught by this unusually honest and fair dictatorship, why not bury the books in an obscure location only two or three inner-circle book people even know of?
These two points, separately, seem to poke massive holes in the plot of Fahrenheit 451°, but then I realized that it makes sense after all - and my theory makes the movie even more awesome. The solution is simple. The book people are agents of the government! Not all of them, mind you. But the idea is simple and effective. The government burns books, but the people might draw hope from resistance. The best way to deal with this problem is to provide an outlet for this hope - a false hope that actually leads to a dead end. That's where the book people come in. Maybe the book people were a genuine grassroots cult that got infiltrated, or maybe it was started by a government agent to begin with. Either way, it grew big.
The idea is, of course, magnificent. People carry their illegal books to the book people, where they are presented with this hare-brained scheme to "preserve" knowledge. Through peer-pressure initiated by the government agents inside the "book people cult", objections to the idea of memorizing books only to burn the physical copies themselves are silenced. Eventually, people are conned into surrendering their books to the book people - books which might otherwise have remained hidden in cellars for ages, possibly reproduced eventually. The agents destroy the books, and the legit book people themselves are kept isolated in their colony, where the agents can keep tabs on them and prevent them from staying in contact with rebels in the city. Presumably, there are similar colonies all over the country.
Once again, I must stress, this is the smartest plan I've ever seen in a movie that was directed to establish a dystopia. In fact, it seems that within this movie universe, the government does actually have a good chance to utterly destroy a large portion of books with this scheme - and the common people will be unwittingly complicit in this. Masterful.