@Chonky and @eishiya: thank you for pointing towards how wrong the shadows are, and sorry to the late reply, I was out of town these days! I was never trying to be too accurate with the shadow direction, but you are right. The idea was that I had a hard light casting shadows coming from the north and a softer/reflected front light illuminating the faces of objects, with equal falloff on either slanted faces of diagonal walls. However I'll need to check reference, I was under the impression that a north light would make the slanted walls cast shadows in this situation. Thanks again for pointing that!
Edit: uh, looking again, if I were to consider a single light source for this, it would be coming from the south and more from above, to give those highlights, so the shadows would actually be behind the objects and not visible at all
@Chonky Pixel, yes that kind of subtle texture around the shades areas and randomly splashed as texture on the main surface is indeed what I plan after eishiya suggestion. However, I find it rather difficult to choose where to texture and where to leave it blank to achieve an organic look. To me it seems to be like a particular kind of skill that lack, and I mentioned in a previous thread that it feels like "Zen gardening" to me, and I am awfully bad with it :/
Sorry for not posting updates right now, I have something soon, and thank you very much for all the attention and critique
Edit: tried my best to use all input mentioned here. I tested generating shadows with a north facing light and the slanted ones did cast shadows, but I think my shadows were too long. I also switched the grey for another one in the palette which is a bit more greenish to simulate getting some diffuse light from the walls. I hope the books shelves look more caved in and the books more 3d. I tried some natural/random detail placement, too much?