I didn't so much learn from books but hands-on. Get VirtualBox (or something similar), install your favorite flavor of BSD and try to set it up just the way you want the server to run. When you run into problems: read up on them.
Basic reading that I'd recommend: IP networking basics, Shell commands & scripting, sed/awk/perl (just enough for some simple tasks at first)
OTOH, I'm learning by that method for about 10 years now, and still learn new stuff almost daily. System administration simply is too broad a subject to truly master.