Story behind the animals on O'Reilly book covers
oreilly.com
oreilly.com
Seriously though, all the royalties go to charity. I'm a bit sad how quickly portions of the book went out of date. I've been asked to do a second edition but probably don't have the time to do it given startup commitments.
They are not only recognizable in a "brand" kind of way, but they are also professional looking, simple but with a twist of the creativity that differentiate programmers from other technical professions.