Arguably, digital stuff might have a chance of being more prevalent than books made of paper, but the places they are stored might be a hell of a lot smaller nonetheless. Then there's just other forms of physical recording: Clay tablets, scrolls, inscriptions, letters carved on tree bark or written on leaves. I imagine there is an archive of gravestones somewhere in the library, and a gigantic city of monuments on which stories were inscribed.
That last one ought to make you think: Can architecture tell a story?
Why, yes, it can, of course.
You might get the feeling I'm advocating for a very broad definition of what the library contains.
Why, yes, of course I am. That's way cooler than having it just contain books.
What about perishability, btw. Books are perishable. They only survive for so long, as does stone, or anything really. That means that non-perishibility is, seemingly, not a hard requirement for getting something into the library.
If that's the case, what about a story that's been handed down orally? Are there vast stretches of the library, filled with the disembodied voices of storytellers everywhere? After all, what is a spoken thing, if not a very perishable record made in patterns of moving air?
There also, arguably, shouldn't be a limit in scale. What if a civilization of gods records or tells stories, by creating worlds in which various complicated ideas and concepts unfold through similitude? Why shouldn't those be somewhere in an endless library?
Place no bounds on library contents; embrace the madness! A report made in morse code via artillery guns? Hell yes! Vast quantities of stages and theatre people in a never ending loop of performing the same play/improf performance/historical event? Why the f* not!?
This ain't your small neighbourhood library! We've got a giant caterpillar, a drunk octopus, and a legal code that is mostly based on temporary hive mind integration; let's not be shy in asserting the vastness and diversity of our Library.