You'd be best served by deciding which distro you want to learn and getting a book specific to that. If you're interested in the corporate world, you're probably looking at RedHat for servers (that's not an endorsement - I personally use SuSE) or maybe Ubuntu for desktops. Fedora is also very popular. Most of the basic commands are the same across the flavors, but the experience of installing and configuring Linux varies widely depending on the distro.