[kkylin]

Looks like the Wikipedia page on CTSS has a number of links:

https://en.wikipedia.org/wiki/Compatible_Time-Sharing_System

One should also know about ITS:

https://en.wikipedia.org/wiki/Incompatible_Timesharing_Syste...

And of course, before Wikipedia there was the Jargon File:

http://www.jargon.net/jargonfile/c/CTSS.html http://www.jargon.net/jargonfile/i/ITS.html