Generally all O'Reilly books have the name for the animal featured on the cover somewhere in the book. However I faced a situation today where this was not true. The reason was that the book I had was the pocket reference for the vi editor, which did not explain the animal on the cover. This must have been due to the fact that this was a companion book to "Learning the vi editor" and the animal featured on both the books were the same.
As usual my curiosity was piqued and I wanted to find out more. My initial feeling was that it was a lemur. However I was wrong and in the process found out about a completely different animal - a tarsier.
A day well spent!!! :)
The main link of this article will send you to the article of the lady who started this trend.
Also visit this link for info about why some of the O'Reilly book do NOT have animal covers.