Anyone who has even had the slightest exposure to programmers knows about the O'Reilly programming books, all of which have woodcut animals on the front. Some of these are so well known, you can just say "the camel book" and everyone involved knows you mean Perl.
Personally I've always loved these books. And their covers. They're reliably good content covering an enormous range of subjects, usually written by acknowledged domain experts.
But why a camel? Why a rat on the python book? Is the playtpus on the webapps book a commentary on the bunged together out of disparate bits nature of most web applications, or is it just that someone thought he was cute. I know I do.
Amanda Pickering @ The Millions apparently asked these same questions, and investigated.
By the way: I love The Millions. It's like booklover crack. And I am your dealer muhahahaha.
(So I just realised, that post isn't actually ON the millions, but it was them that tipped me to it.
So go read this too, for instance: http://www.themillions.com/2013/02/judging-books-by-their-covers-2013-u-s-vs-u-k.html