Re: Best manual
That wasn't just a manual, that was The Coherent Lexicon. Possibly the best over-all operating system book ever published. It is still a valuable tool today, with the caveat that the details of CLI commands have changed, but that's easy enough to check with modern man pages.
It is available for the download here, but I suggest finding a dead-tree copy on fleabay or the like. I've also seen it in used bookstores, mainly in University towns. Why dead-tree? Because it doubles as a reference manual, and to date computers don't come close to actual books when looking up shit in a hurry.
Couple the Lexicon with a copy of ORA Power Tools, 2nd Edition (the Drill Book) and you've got a really, really good starter's kit on REAL system administration ... but again, with the above caveat.
You can also download & play with the actual Coherent OS, should you want to. A legally downloadable copy is archived at TUHS, here.