I use terminal to log in to my remote linux boxes to maintain them. There isn't much I need to do on OS X in the terminal. The most recent thing I had to do was to re-enable ftpd (ftp server) which Apple suddenly decided we don't need any more. Well I'm not about to stop using my scan-to-ftp scanner so I went in terminal to get ftpd running again. I realized a lot of users might have this problem so I made an app to enable ftpd. It's on MacUpdate and linked in my sig.
There really is no compelling reason to use the terminal except to learn about Unix. As for books, I would recommend getting used to reading "man pages." A lot of documentation ships with Unix (and by extension OS X). For instance, start by reading up on the bash shell after a google search. You come across a command line app you want to try like grep. You type man grep in terminal and you get the (rather primitive) online help. From there you can search for more on the web. In fact I'd say for learning Unix, the web is more useful than books. But I must remind you: Be careful! The terminal is no place to type things and see what they'll do. If you aren't careful in terminal, you could wipe all your the files in your home folder or render your Mac so it won't boot.
If books work better for you, ORielly is an excellent source. One in particular called
Learning the Bash Shell is under $20 at amazon and is even $5 cheaper to read on a kindle or in the kindle app on an iPad or on your Mac.