[Brutha]

I mean, it helps to know what the limits of something are, to see what the experts can do with it.

But then you shouldn't look at something who is in the business of creating the impression of impressive mental effects.
Derren is a bad source when you want to know about the limits of something. You don't know which effects are slight of hand, which are hypnosis and which use different methods.

Really? That's kind of suprising to me, as what he does seems pretty straightforward - he is a magician using his skills to fool people, and he is open about using tricks and manipulation.

Yes it's interesting how people think that someone who is open about using his skills to fool people gets a result that someone like you think that they haven't been fooled.
It's the classic stratagem of hiding in plain sight.

In this particular experiment, it would seems any trick would be totally unimpressive, since anyone can put a camera behind the librarian and see what he is reading, or some other trick.

If you know the trick of a magician it's often unimpressive.
If you however don't know the trick it gets impressive.
That's what magic/mentalism is about.
Using a hidden camera is a trick.

Derren show starts with announcing that there no stooges used.
He doesn't announce that he doesn't use camera editing and hidden cameras with are both in the standard tool set of the trade.

It would be just dumb and would require no skill, while Derren's tricks are usually pretty clever, at least I think so.

That's what it's about, using a variety tricks to create an impression that it's clever.
A lot of people like yourself can be easily made to believe that they have seen something that's pretty clever by using tricks and manipulation.

How about going through 1 page at 1 paragraph per second?

I don't think that you get what photoreading is about.