Perl Template Toolkit, or the Badger Book, as we like to call it, is a comprehensive guide to using and extending the Template Toolkit. Here's some blurb from the back of the book:
The biggest cause of unmaintainable web sites is mixing markup code such as HTML and program code such as Perl. The best way to separate them is to use a templating system. The easiest and most powerful templating system for Perl is the Template Toolkit. Perl Template Toolkit is an introduction and reference to the Template Toolkit for programmers, web designers, and anyone else who has ever thought "there has to be a better way" while debugging a tangled web site.-- Thus Spake Badger
You can read more about the book and browse through it on O'Reilly's web site. You can purchase it at all the usual online stores.