That all depends on yourself. As a *book*, Mastering Perl/Tk is fine (and you don't need Lerning perl/Tk). As a learning path, you can just install Tk, and start with viewing the script "widget", try to understand what it does and read the on-line docs (which are way more up to date than any book curently available).

As a second "booK', I'd like to point to Perl/Tk Pocket Reference, a valuable addition imho.