Coherent was a Unix clone operating system for PCs that was somewhat popular in the late 80s and early 90s. I have fond memories of buying a copy via mail-order, probably from an ad in the back of Dr. Dobb’s, and probably for $99, and using it on my PC at home. In the days before Linux, Coherent was a great way for an individual with a modestly-powered PC and a few bucks to spare to learn all about Unix. Coherent came with a huge manual. (An actual book, printed on paper. Not just a PDF or a bunch of text files.) And it was fairly well-written and well-organized. You could really learn a lot about Unix by reading through the introductory material in that book, then messing around with things on your PC, then going back to the book for reference.
Just recently, the sources and documentation for Coherent were published on the web, including that gigantic manual. I had held onto my copy of the manual for years after I’d stopped using Coherent, just because it was such a good general reference, but I finally threw it out some time ago. Well, now I have a nice PDF copy if I ever need to refer back to it again! I’m tempted to try and get Coherent running in a VM on my current PC, but it’s probably not worth the bother. It would be kind of fun though.