In the library, there is a "hidden door" / "secret door" in the back corner (at the end of the bookshelf). There is a beckoning hand cursor when you go over it, but when you click it, there is only an animation of the wall changing into a door and back tot a wall again. Is there more to this than just the animation? Or can you unlock something so it actually functions as a door to another place?

I have the same question for the "geared" door in the Laboratory. As far as I know you can only leave through the door with the bars, but there is a second door with gears. When you click it, you see the gears moving, but that's about it...

Yes, there's something more to the fireplace door--however, you cannot access it until first you have explored the area the door in the fireplace connects to. Once you have, come back to the fireplace door and you should be able to proceed through it.

The gear doors I imagine work the same, although I can't say for sure as I haven't gotten there yet...;)

A very late response, I know ;-). But I was not talking about the door in the fireplace. I know this is a transit. But I'm talking about the spot right besides the bookshelf.

Both doors are just animations, unfortunately.