It's hard to measure the information content of anything, because information is fundamentally about differences which matter, and we don't always know what matters. The text content can be preserved dutifully through centuries through copying, then in our time, we find out that what we really would have wanted was the handwriting style of the original, or the environmental DNA from pollen attached to the original vellum...
But even so, there's so much archive material which hasn't even been digitized. I run into it in genealogy all the time. It's in some box in a museum, if you're lucky they made microfiche images of it fifty years ago.
When you add up all the books that were required for our careers, would they be a megabyte?
The little that we understand is uncomfortably summarized this way.