That you could buy a book, read it, and then know literally everything about a particular system. And often that book came with the system.
They must have been well-written; I didn't really have Internet access, so there was nowhere else to go if I got stuck. Though I was probably a much more determined learner back then.
Someone here mentioned StackOverflow, but really, a handful of manuals, maybe a good book or two, and some persistence, /usually/ gave you all you needed to figure out a problem, and do something magical.
Now, those resources (if they're available) will likely only help you understand a very small subsystem.