One of the bookshops I haunt has a specialist area where they sell textbooks and the like: whenever I visit that section, I sually see an array of programming textbooks from O'Reilly Media. Given that these books are usually about the complex minutiae of various computer languages, database implementation and other coding problems, it must be hard to come up with dynamic cover images that represent the topic. So I have to admit to admiring the way they just said, "That's impossible, so fuck it, let's put animals on them all!" (this is not sarcasm, by the way--these books really do stand out from the other books on the same theme, which usually have abstract CGI blobs or stock photos of young people looking amazed at monitors).
UPDATE: Designer Edie Freedman talks about the cover designs here.