you want some administrative knowledge then read "Linux in a Nutshell" or 'Linux kernel in a nutshell' (http://www.kroah.com/lkn/
or just the basic knowledge then, some magazines based on linux, books like 'for dummies' series, as mentioned above, forums - like this one, also Linux Complete reference and/or a book just for the distribution [ also known as distro ] of the one, that you're using [i.e. opensuse, ubuntu, redhat, fedora... etc
and finally, you've to start using it.
A piece of advice, that I got late:
Linux, itself is as big and sound and deep as a sea. so try to be specific in terms of your field of interest.