The TeXbook, and the Exim spec. The thing the latter gets right is that, as well as the usual long list of options with an explanation of what their effects are, it also describes in full detail what the program actually does, rather than leaving you to deduce it from the options. So each option is normally described twice: once in an alphabetical list, and once where the operation it affects is described.
More documentation back then seemed to be of higher caliber, than much if not most of what we wade through, today.
I think the people writing it had more time and resources for attention to detail. And perhaps more need. There was no generally public Internet for updates and corrections including turning your product from a pain in the ass quasi-brick to something useful. You had to get it right, up front.