I used to be a bit of a Perl hacker and had 4 different O'Reilly Perl books. I found that if I didn't write any Perl for a couple of weeks I would quickly forget the cryptic syntax and had to go back to the books when I next wanted to write some Perl.
After a sabbatical from writing code I was faced with a challenge of writing a script to process some text. I could either use Perl and almost have to relearn the language from scratch (I'm sure it was designed to leak out of your head!), or try something new.
I decided to try Ruby.
Armed with only a few websites for reference (including the Pragmatic Programmers' online book) I quickly cobbled together my script in far less time than if I had used Perl. It was so much easier!
Now I will never go back to Perl and have subsequently sold my Perl books (sorry, Tim). What's more, with the free online resources I am unlikely to buy a book on Ruby.
If my experience is anything to go by, then I would suggest that the number of Perl books sold is not a good indicator of the popularity of the language; it's an indicator of how much you need to resort to books to actually use it.