I own quit a lot of books on Perl. But when I look on my desk or bookshelfs the only ones you don't want to touch without rubber gloves are Mastering Regular Expressions and Camel 2. Look at this books and you will know they had a hard time and traveled a lot.
If you are not one of the "Wash your hands before you thouch the book" & "Allways treat books nicely" pedants you will know which are your most valuable books just by looking at them.

In my opinion this book is one of the most exciting, helpfull and entertaining computer books I have ever bought. And it is not difficult, but easy to understand. Just 2 weeks ago I gave it to a linguist colleage (without programming experience) of mine and even she got excited on Regular Expressions while just using different dialects of grep.
Its not just a Perl book, but The Survival Guide for people with too much Data.

Hanamaki