Actually, I don't believe that you'll learn more from books than you will from solving problems to questions here and on other forums like ubuntu.org. Get stuck in and put some skull sweat into it, and you will be surprised how much you will learn and how quickly. Google and other search tools are your friends, as well as being willing to experiment with your own system.
Get some books by all means, but they are references and primers, not the only thing you need.
While we're looking at books though, The first few on this page look like they might be worth a look. There's the IBM pdf bash primer available here which I've used in the past, and here's a bash scripting primer which I haven't come across before.
Googling keywords such as "bash primer" should show up many more hits.