One of my favourite conceits is that writing is about the human condition. Any and all writing.
If it's a mathematical text for university students, it tells what is current accepted knowledge and theory in that particular sphere of study.
If it is tabloid media, what are perceived to be the opiates of the masses (male/female/trans/bi/a - all have their own particular brand)? And this is the Roman circus for each age and culture.
If it is romance - what are the social mores of the time and place, what makes this an escape from reality? Is it written as pulp or as literature (cover will generally tell you this!). What is considered desirable....
If it is fantasy/science fiction (they can be distinct, they can be interwoven), just how far have we come, how much is envisioned, what are the hope and fears for the future from the point of time in which the story was conceived and written?
But it's not just the direct content. What is written and how is also a reflection of the writer, their times, their influences, their social context. Sometimes, that can be more interesting than the writing itself.
How it is presented can say much as well. Books really took off as mass media with the printing press. Before then, it was a laborious process of copying to vellum. Wealth and status came with an elaborately illustrated and bound library of a few hundred books. There was also the original tablet - a wooden cover with wax in one side, ready for ephemeral notes. Paper - well there's the old rag paper, made of cloth, which lasts incredibly well. Also when made from fibres like flax or hemp. And there's wood based paper, which we take for granted and becomes brittle with age. But still, how it is printed and bound still tells of the time and circumstances in which it was made. Paper used before WWI is different to the paper used in austerity prints of and after WWII, and different again is the paper from behind the wall that used to divide Germany, while there was still a USSR.
If the edges are rough and uncut, there's a printer, boasting of how carefully he aligns the paper. If the pages are folded and need to be carefully cut open, that's another print/bind method (edges to the binding) and that poor book languished unread until it reached your hands.... The fonts used, the point when the long s ceased to be used, gilt edging, marbled end papers, illustrated pages that had tissue paper in front of them, bindings that were cloth, leather, paper, board, limp, stiff, elaborately decorated, dustjacketed - all of these things say something about the human condition.
And now - now that books are comparatively cheap, a library of several hundred books is neither here nor there. Now that e-readers are available, so much can be held on one device that bookcases could be rendered superfluous. Despite the resources needed to make and run such a device, it can work out better ecologically than mass printings. They certainly make small print runs of academic texts much more affordable (always a consideration for perenially broke students). That in itself says something about our world, about our condition at this time.
But that doesn't stop a bibliophile like me from continuing to add to my library.