This is quite a good idea. It gives their line of books a uniform appearance while still giving each book an individual identity, with the CamelBook as the prime example. Similar is WroxPress using photos of authors on the covers.
O'Reilly has posted a few essays about their use of animals on book covers: http://web.archive.org/web/20090226012506/http://letters.oreilly.com/pub/a/oreilly/letters/2003/animal_index_0203.html Date: February 2003
Don't you love the way they had a perfectly good animal given the title of this book, but completely disregarded it?
There is a reason: http://www.rmi.net/~lutz/lp-cover-post.html Wood rats must learn about Pythons!
Alternatively, maybe this is why:
The "Learning" books often have smaller, cuter animals than other books. Compare
with
and
I'm not sure there's such a thing as a cute python, but a cute rodent is easy.