|P is for Practical|
Mastering Regular Expressions, by Jeffrey Friedl is so good, I just don't see how anyone can do without it if they're using regular expressions at all.
If you've ever been tricked by greedy quantifiers, lazy regexp engines, confusing lookaheads, backreferences, POSIX character classes, inline eval, recompilation, sluggish alternation, and so on, you need this book. If you don't recognize some of those terms, you really need the book. I know that the Perldocs are touted as being the definitive source on Perl, but in the case of Regular Expressions, the Owls book has got to be the standard work.
Every time I see regular expression related questions posted to Perlmonks, Usenet, or anywhere else, I think, "Why didn't this person just read Mastering Regular Expressions?" It's the book that turns run of the mill Regular Expression authors into Masters of the Craft.
When you read it, be prepared to start seeing everything in life in terms of regular expressions (except for irregular things, of course). And be prepared to start counting to ten a lot before posting replies when you see questions on Perlmonks.org by those who obviously haven't been to the fountain yet. You can spot them immediately. It's ok, your work is easier than theirs because you took a few days and read this book. Go easy on them.
If there ever were a mountain with a guru sitting atop in a secluded hut, Friedl would be that guru, and the otherwise arduous journey to the mountain top would be facilitated by the Owls book like an air-conditioned tram car. The journey back down, of course, would be in the slipery slide of an efficient regular expression.
"If I had my life to do over again, I'd be a plumber." -- Albert Einstein