As an old-timer, I worry that it may have been forgotten that all the major systems vendors used to have extensive shelves of high-quality technical manuals for their software, written by their in-house teams of professional technical writers. And many of them were really good, especially the ones on programming languages. The Univac APL 1100 manual might be the best book on APL ever, for example, and the CDC Fortran manual was a superb reference. I wonder if any of these old gems are available on-line.