I suspect this is at least partly because the Camel book (Programming Perl) was one of the really early Perl books and, because of who wrote it, also very good.

Thank you. It was a great project, and I'm glad to have helped make it a historical event, although at the time, it was just a way to get some more instructions down for Perl users, using my combined skills as an award-winning technical writer and editor as well as being a programmer from nearly birth. {grin}

But truly, O'Reilly was (is still?) the only publisher run by people who started as technical writers, so they're much more inclined to let the smart authors do their thing and concentrate on all the rest of the stuff an author needs to be successful.