"As the author (and director) sees fit." - At least that's been what I've kept telling me.
This is not just related to reading, though.
Just look at the general promise of the Doctor being present somewhere - why?
- Sometimes he's somewhere just because he wanted to be there (visit something with a companion or whatever).
- Sometimes he's somewhere because he screwed up again or the TARDIS wanted to be there. There are some episodes where he says something along the lines of "that's why the TARDIS brought us here".
- Sometimes he seems to know in advance why he has to go somewhere (like in the 2005 pilot episode; although I'd consider this up to debate, because he might have learnt of the invasion by accident before the start of the actual episode).
Or how about reading and interpreting something?
- We know the TARDIS translates voice as well as text for those being around.
- Yet there are instances where the doctor or the companion(s) read something and don't get the actual meaning due to interpreting something wrong. Considering that you can't translate most real world languages without proper knowledge of the context, this sounds rather odd as well. If you think about it, it would happen more often than just a few times that they'd babble or read random stuff without it making sense, similar to Donna trying to speak Latin when visiting Pompeji.
But back to the actual question:
Why would it be determined by the author or director? It's rather simple.
Seeing the Doctor (or companion(s)) read something can have different effects on the viewer (and in the end it's all about that; The last episode with the Ponds in season 7 is a perfect example for this):
- It can build tension (takes a while to read or something is read slowly).
- It can just explain knowledge about something or be used as a plot device.
- It can be used just to distract the character so something may happen in background.
- It can be straight up boring.
Especially the last point is important: Imagine the first episode of the 2005 reboot and seeing the Doctor looking through the book for something like 5 or 10 minutes without anything else happening. That wouldn't be interesting or anything. So instead they let him dig through the book(s) a bit, similar to how you might have a look at a book you see in a book store. Still this doesn't mean he's read the whole book that exact moment. He might even have read it before and just refreshed his knowledge looking through it.
And one more note:
[...] he just seems to be surprised by everything you'd have seen coming.
This is actually a rather common TV/movie trope, unfortunately, especially in horror movies, mystery stuff and sometimes thrillers as well. It's that bad old "no, don't go in there, you're going to die" thing, where the characters still do it, sometimes even ignoring common sense and the like. That's not really Doctor Who specific.