Like I said before, the internet is full of usefull Linux information. A book is good since it's nice to have a reference close to you. Say you put in a new ethernet card, but forgot a command to get the drivers installed, so you can look it up in the book since you don't have access to the internet.
It also depends on how you read and learn stuff. I had a friend that when he read any sort of reference book, math, cars, or computers, he would highlight everything and use tags to mark important stuff. Me on the other hand just kinda flips around....
If you want to learn the comand line (actually the proper Unix word is shell) you can find all sorts of tutorials, references and howto's about anything.
For instance here
Another thing you can do that's interesting is sign up for a shell account. Since the internet is mostly Unix it was very common for people just to allow anyone to log onto any computer via telnet. As crackers and script kiddies got more popular, this has died off, but it's still fun sometimes to send a dollar or 2 in and get a shell account on some internet server. Some ISP's still have shell accounts as part of their service, even if they don't advertise it much. There are still many free shell accounts out their, but often it is limited in what you can do on one.
A popular one is
this. They have a limited shell for free, but once you confirm your identity (issues with crackers stopped anonymous accounts) by sending in a dollar in a envelope with your address you can get full access to a shell account, plus you get some webspace, too. It's nice to mess around it, and has forums and stuff for asking questions and stuff and is nice for testing your internet services from something outside your own firewall.
You know the internet isn't just all email, chat, and advertisments!
edit: But then again, with modern Linux, if you don't feel comfortable with a Shell, you don't have to use it all.