New industrial printing techniques meant you could print lots of books cheaply; a modern capitalist marketplace had evolved in which you could sell them; and for the first time there was a large, increasingly literate, relatively well-off urban middle class to buy and read them.
Jumping forward to the 21st century, and the rise of (among other things, ebooks and e-readers) the article makes a statement that matches what I've said several times:
A lot of headlines and blogs to the contrary, publishing isn't dying. But it is evolving, and so radically that we may hardly recognize it when it's done.
At another point, the article mentions the change from self-publishing being seen as the height of vanity to becoming simply the farm system for getting a book to market:
In theory, publishers are gatekeepers: they filter literature so that only the best writing gets into print. But [three successful self-published authors mentioned by name] got filtered out, initially, which suggests that there are cultural sectors that conventional publishing isn't serving. We can read in the rise of self-publishing not only a technological revolution but also a quiet cultural one—an audience rising up to claim its right to act as a tastemaker too.
What the author doesn't mention here is that not only is taste a subjective thing, but the move to more and more conglomeration—publishing houses merging to save labor costs—has meant a tiny group of editors are making the decision over what gets "published." I consider this one of the primary reasons so many good authors are trying self-publishing—not just because technology makes it cheap and easy, but because there's just nowhere else to go.
The article makes some interesting predictions ("Novels will get longer—electronic books aren't bound by physical constraints—and they'll be patchable and updatable, like software.") and maintains its optimistic tone to the end.