There are too many books in your campaign. I mean I hate to presume, but your campaign is probably littered with books. There are probably libraries on every street corner, rich people’s houses full of bookshelves, maybe even a character class that necessitates owning a book? Does this sound familiar?
In early medieval Europe, for two centuries, 120 books were produced every year. Not 120 different books being written - fuck no - 120 book-objects were completed each year during the 6th and 7th centuries. Mostly bibles. Some years - when Vikings hit a particular monastery, or just when some novice knocked over a candle - this must have been below the replacement rate, and the year must have ended with fewer books than it began. Not to mention that half the time books were produced by erasing old ones and reusing the paper.
Now, being fair, this didn’t last (you know, more than two hundred years). Even before printing happened, the 15th century saw 5 million books created. Even in the 10th century - still extremely the Dark Ages - there were 2,000 books being produced a year (but then this number collapsed again in the 11th, when the Vikings and the Magyars arrived on a mission to reverse historical notions of linear progress). But 2,000 books a year in the 10th century, and maybe 10,000 a year in the ‘High’ Middle Ages of the 11th-13th, is still not that many, especially giventhe area we’re looking at: see below for my source but we’re talking here about all of pre-Reformation Catholic Europe, which is almost certainly a far greater area than your campaign is taking place in. The time and place where the Middle Ages ‘peaked’, 13th century France - still a bigger area than your players will ever thoroughly explore - was producing maybe 5,000 books a year, Germany, Britain and Italy perhaps half that many. And although that kind of thing adds up over the years, it doesn’t add up that much, especially when most of the books are bibles distributed in individual churches: in 1289 the largest library in Europe (at the University of Paris) had about 1,000 books in it.
The mass availability of the written word is one of the defining aspects of modern culture and society (we ourselves have lived through the moment where the written word became essentially infinite), and almost all D&Dery (broadly construed) is aiming for a pre-modern society. Yes, there are obvious good reasons for books being relatively common in campaign worlds (and there are also clever ways to introduce more ‘books’, as with Vornheim's snakes), but sometimes it’s just convenience and laziness. Where are all these books coming from? If your campaign world has the printing press then this is a simple question and lots of books is fine, but if it doesn’t then this question poses a problem. Does the (admittedly intentionally vague) 5e version of Faerun have printing presses? Have the writers even thought about this? There’s something much longer to be written sometime (probably not by me) about whether the general underlying assumptions of D&D are medieval or Renaissance or a bit of both and whether that mix of assumptions makes much sense, but it’s certainly the case that the availability of the written word in most D&D is right at the top edge of plausibility for any kind of pre-modern society.
Not, obviously, that historical accuracy is the point here: the point is that scarce books can transform a campaign for the better. Much of what I said here about bells can also apply to books when books take a year to make and are worth hundreds more than a peasant earns in a year. Any book can be a Book of Vile Darkness-level instigator of conflict and quest, and the handful of extant libraries (which is to say, places where there are more than a dozen books) can be genuinely valuable locuses of learning and magic. Scribes are important figures in society, and their services are valuable (I can’t be bothered to get into the question of literacy among PCs, but, yeah). Possibly most interestingly, there is lots and lots of knowledge that only exists in one place: spells and chronicles and maps and confessions which, once destroyed, are gone forever.
Books in your campaign should not be like apps for your players, a suite of easily consulted adventuring aids and plot coupons. Your campaign needs evocative, rare, fragile, valuable, versatile, powerful, interactive items, and books are them.
Obligatory note on sources – my source for all this is here https://socialhistory.org/sites/default/files/docs/projects/books500-1800.pdf in this completely fascinating paper. Obviously (as they note at length) these figures are very hard to establish but this is the best possible effort. Other model societies are available, obviously: Byzantine and Islamic societies of the same period were probably much more literate.