I don't recommend any books on Linux. The best information you will find is freely available online. You are better off to be distribution specific if you buy a book or seek information online though, because not all of them are implemented the same and generic information you'll find isn't all applicable for you, and you may waste considerable time not realizing that. Generic Linux information is more useful when you already have a firm grasp. Google is a very useful tool in addition to collections of links, like the site Slohands posted. Google even has a specific search for Linux information: http://www.google.com/linux

If you're going to purchase a Linux book though, ensure that it's written by the authors of the distribution or software. If anyone deserves to make money it is those that actually did the work.