One doesn't learn GNU/Linux from a book....one learns it from using it. HOWEVER:
Linux Server Hacks
Linux Networking Cookbook
Pratical Guide to Linux: Commands, Editors and Shell Programming.
IBM developer works
Someone said things change rapidly on GNU/Linux. I disagree. Ancient programs run just fine, conventions used a decade ago usually still apply, etc., etc. The only thing that changes rapidly in the GNU/linux world are desktop environments and the kernel. But, kernel changes don't preclude backwards compatability.
You may install Gentoo and or ArchLinux for an education, too. You will be forced to learn the system to a certain extent and then those books I mentioned will be handy.
Keep this in mind about GNU/Linux -- everything is a file. This is handy beyond belief. The bash shell (your 'terminal') is where it's at.
Stay away from Ubuntu, it's problematic once you start doing actual things on it.
Just wait 'till you see how fabulous GNU/Linux is from the admin point of view.