Before I read this book, I would have agreed that printing was a hugely important technology. But I had not thought very much about the details of where that effect was seen.
This is a long detailed work of history and historiography to trace why printing mattered, and to show that previous historians had understated its importance. The book is very much written to an audience of professional historians, but I was able to follow it and was enormously stimulated.
Here are a few examples of points that Eistenstein made that I hadn't thought about before.
Scholars in the late middle ages and early modern period were fully aware that there were ancient books that had gotten lost but might reappear, full of philosophical insights and lost technical knowledge. Given that background, it's no surprise that many scholars were fascinated by the occult and hermetic literature: if you didn't know any better, you might think that the lost wisdom of the Atlantean masters was just the next thing that would come back after De Rerum Naturae. Before the 14th century, there wasn't quite the same awareness just how much had been lost; you would know that your monastery didn't have something, but you wouldn't have any way of knowing whether it existed elsewhere. After the 17th century, you wouldn't expect a lot more to surface, and you'd be much more aware of the possibility of fraud.
I had never focused on the way printing revolutionized science and mathematics. Before printing, it wasn't possible to reproduce technical diagrams or large mathematical tables with sufficient accuracy; texts were copied by monks, often based on dictation. The monks were typically not technically expert, and so the illustrations were often bad or absent. In contrast, with a printed version, you could steadily improve the plates. As a result, early modern copies of Euclid were much clearer than medieval copies, and technical writers like Agricola or Vesalius could be understood much more easily.
The usual historical narrative is "Ptolemaic Astronomy went unchallenged for a thousand years and then Copernicus offered an alternative." As Eisenstein points out, hardly anybody in the medieval world had a copy of the Almagest, most of those copies wouldn't have been very good, and even if you had that, you probably didn't have many other books. Copernicus was in the first generation of astronomers who had a copy of Ptolemy, had copies of Aristotle and of ancient books with astronomical observations; as a result, he was among the first people who actually could notice some of the problems with Ptolemy. Moreover, the succeeding generation of astronomers (notably Tycho) were the first to have *two* different major astronomical works to choose from and compare. Before printing, you wouldn't have noticed the problems with Ptolemaic astronomy and wouldn't have had an alternative, with different problems, to compare it to and stimulate creativity.