|P is for Practical|
Are paper and ink books an irreplaceable resource in the journey to enlightenment that is Perl?
Irreplaceable? No. You can learn Perl just by reading the perldocs and whatever you manage to find online, if that suits you. However, learning methods are subjective, and for many people a book has a usefulness that is hard to replace. Think about practical issues: you can read a book when you sit at your desk, or in an armchair, in a train seat, in a airplane seat, in bed, while walking, even in your bathroom. Now, you could do some of the above with a light laptop, until you run out of batteries, but mostly, reading from a computer screen for long time is more stressful than reading a real book.
Electronic reference is better than paper, because you can search it quickly. That's why I have bought the Camel book both on paper and on CD. :)
Does the Camel book, in particular, contain something that is not duplicated in perldoc or the collective knowledge of many questions asked and answered?
Yes. The footnotes ;)
Seriously, though, the book is a masterpiece. It does teach Perl without the dead seriousness that other reference manuals have. It's funny, informative, detailed, deep, almost complete (related to Perl 5.6 that was the current version when the book was printed), and most important, it addresses the reader as an intelligent person, not as a dumb moron who must find where the ON switch is.
The Camel book is a book for smart programmers with a sense of humor. It's a book for people who want a confrontation with a Wizard Programmer who talks to you as a peer, rather than a condescending professor who treats you as a helpless case.
If you are a smart Perl programmer, get a copy!
P.S. I don't get any royalties for this advocacy. :)