[jpluscplusm]

As a sysadmin who wishes he'd discovered this book earlier, I have no problem with recommending "The Practice of System and Network Administration" as a great resource: http://everythingsysadmin.com/aboutbook.html

It's definitely an OS-agnostic book, so if you're looking for "how do we do task/process/design A with OS B?", it's not the right fit. It'd still be worth it if you're expecting to have to do anything except the very shortest-term systems administration.