The regular old perl TK module is not "dead now" -- it is still being maintained. I would guess it's one of the first things many people who learn programming via perl make use of, it is not particularly difficult or awkward as far as GUI stuff goes, I've done great things with it and I am sure plenty of people still use it.

You should understand that TK and TKx are both just perl interfaces to the same system Tcl/Tk library. In other words, if there is any "advantage" of one over the other, it is purely in terms of the style of the perl interface. From looking at the Tkx CPAN page, it appears the author simply wanted one which presents more transparent access to the Tcl library. If you are not a Tcl programmer (Tcl is another interpreted language like perl) then that may be somewhat irrelevant for your purposes.

To put it another way: this is the same as the difference between doing (eg) a GTK application in python vs. a GTK app in perl: the end result will be identical and uses the exact same system libraries. They are both GTK applications. So, if you have a good book on the TK module (I also prefer books), just use that as I suspect you will have to wait a long time before you find any books specifically on TKx (like, forever...). This is from the TKx CPAN page:

The main idea behind Tkx is that it is a very thin wrapper on top of Tcl, i.e. that what you get is exactly the behaviour you read about in the Tcl/Tk documentation with no surprises added by the Perl layer.

This makes sense if you want to use the Tcl/Tk (non perl) documentation to learn Tk, which online is probably more extensive. But if you want to learn it out of a book called "Mastering Perl TK" then it is a non issue. Hence, the book is only "no longer relevant" if you are sure you'd prefer the other module for some reason -- do you have any concrete reasons for that? If so, there are books on Tcl/Tk, but that seems very awkward to me as there will be no perl in them. You might as well just learn Tcl...