Well, most of the underlying technology in linux hasn't changed very much. There are some key differences in kernel 2.4 vs. 2.2, but most of the books out these days cover only 2.4 or both 2.4 and 2.2. However, things like command line scripting and administering daemons and what not haven't changed all that much; they still have their roots in old Unix.
It depends what you're looking to do with linux. The books I mention give a good overview of everything, but most likely you will want a speciffic book on Apache, MySQL, PHP, Sendmail, or BIND (that's an inclusive or)... depending on what you want to do.
-Venkman