Not a book, but this online resource is pretty good for learning Tk:
http://www.tkdocs.com/tutorial/index.html. Every example is written in four programming languages: Tcl/TK, Ruby, Python and Perl.
Once you know the basics, original Tcl/Tk code can be pretty easily translated to perlTK. A lot of source code can be found here: http://wiki.tcl.tk/. Tcl, as it is, is not a complicated language, and it's easily read if you know Perl (although they're superficially similar).
Update:Of course there's a lot of perlTK code out there to learn from. My starting point for perlTK was Tcl/Tk.
