Just what it says. The author had read the handbook soon after it came out and has revisited it 25 years later. He also offers interesting comments about most of the competing operating systems. See also
Making old systems accessible via the web https://athornton.dreamwidth.org/14733.html
Exploring old unixes https://athornton.dreamwidth.org/14340.html
A Guide to Decoding Mode S and ADS-B Signals. An open access (CC BY-NC-SA-4.0) book about Automatic Dependent Surveillance-Broadcast (ADS-B) and Mode S secondary surveillance radar systems.