Editor’s notes of the sort you’re talking about were a useful tool in their time. However, in a world in which just about everything we do is collected as books, (not to mention available in digital format), those selfsame notes become confusing and even off-putting to people, because you’re not steering them to the same thing that they’re reading necessarily. I’m sure that the day will come when, at least in the digital versions of our books, we’ll be able to have an editorial note that you can click on to purchase and download the book that’s being referenced. But the architecture of that is still some time away, so that it works for all people on all platforms.