So the Computer Science departent at my university has a shelf where people can give away technical books they don't need anymore. I found a giant UNIX system administration book from 1995 there the other day, and i am blown away by how many useful things i could find in such an old book; basically all of the unix flavours mentioned are long dead (with the exception of bsd and linux ofc), and so is 99% of the software, but all of the core concepts and basic tasks still hold true in 2018... Isn't that amazing? :D Where else can you find a system that still works the same after 23 years?

Any suggestion on (pls, cheap) books that'd enable me to understand the unix/Linux environment in its most fundamental level?

I do use Linux for some 10 years already, but just now I moved to a distro that doesn't do everything to me. So my basic idea is to get the basics so I'd have the least problems when transitioning to other distros/*ix systems, so pls, the less its distro specific, the better or is.
Thx