This raises another interesting question: I was able to find a site that had The Perl CD Bookshelf available online through Google, and Google has at least partially cached the books. I was able to find the full text of more than one chapter.

So, even if the site goes down, Google will still have the cached version available. You would think that they would have gotten in trouble for this by now.

On a related note, I was able to browse MathWorld after the site was (forcibly) taken down by CRC Press, but before the Google cache disappeared.