A very detailed article about the Dynabook was published in 1972:
The PDF makes it sound like an internal Xerox PARC report but it was actually article 1 in ACM '72 Proceedings of the ACM annual conference - Volume 1.
While previous patents are the first thing the US Patent Office looks for in prior art that might invalidate a patent under consideration, an article such as the above counts just as much. A public demonstration of a product would count too. In fact, even if the prior art was produced by the author of the patent it can invalidate it, though there is a grace period (six months or something like that) in the case of the patent's author. I don't know if that has changed in the move from "first to invent" to "first to file" by the USPO.
Once thing that always makes these discussions more confusing than they should be is that there are patents for inventions (how it works) and also patents for designs (how it looks). They are different and how they are invalidated by prior are is different. Since the summary talks about "rounded corners" and stuff like that we might be talking about a design patent.