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