I've been thinking about this...
For most of history in most literate cultures, books were hand-written. Lots of time and effort went into it. So a full-length book was a treasure. Ancient or medieval libraries only possessed a small number of books by today's standards.
So what would happen is that students would study a single book far more intensively than anyone would study a book today. Oftentimes they memorized it word-for-word. (We still see this kind of memorization of "root texts" today in Tibetan monastic education.) Medieval European universities had low level faculty called "readers" who would read a text out loud, while students in the classroom hand-wrote out their own copies for themselves.
This kind of situation meant that educated people read far fewer books than educated people read today. But they were far better acquainted with the text than readers in today's day and age, when people often can hardly remember precisely what a book said right after they read it. Ancient and medieval scholarship was a narrow but deep kind of education.
Then in the 15th century the printing press was invented. Bookshops quickly appeared and individuals started having personal collections of books much larger than even the better ancient and medieval libraries. But many of the books on the shelves were never read or were read with far less care than earlier generations devoted to reading. Scholarship became broader but shallower.
Today books seem to be gradually disappearing in favor of much shorter pieces of text that will fit on a screen. One rarely sees people reading books any longer, a sight that used to be common. Everyone is staring at their cell-phone, and we can be certain that most of them aren't reading books on that tiny screen. Attention spans seem to be dramatically shinking as the broader but shallower theme continues and may be approaching its reductio-ad-absurdem. Where once a student memorized Aristotle's Categories word for word (or Vasubandhu's 30 Verses or whatever it was) today a few sentences seem to suffice in convincing readers that they have mastered the material.
To be frank, I'm not convinced that it's a wholesome development.