While sorting my shelves, I came across a book my dad had around when I was growing up that I eventually grew into understanding (but it took awhile); IBM's 「Technical Reference for the Personal Computer」, published 1983. It's a marker of the cultural-technical shift we've seen since. IBM combined the openness of the days of hobbyist computing with the professionalism and attention to detail that a large tech company would use for its internal manuals. It's shockingly open. You get pinouts. You get circuit diagrams for large parts of the IBM PC motherboard. You get wiring specs. You get a dump of the system BIOS.
If you wanted to write a new operating system for the 8088, maybe another CP/M clone or PC-DOS, you would not need a lot more than this (it documents the 8088 processor too).
How many pieces of computer hardware do you see nowadays that do that? Maybe Apple ruined that when Steve Jobs ruined the Woz's vision, or maybe IBM did it themselves when they realised how much money the personal computer market was going to be (although they presumably thought it was at least worth betting on or they would not have bothered making the PC). I hope we don't lose too much more of this openness in the years to come.