I noted in a previous post how a character from Ergo Proxy was alluded to in Motto To LOVE-Ru and The World God Only Knows, but we can see a more overt reference in another episode of The World God Only Knows.
The title of the book reads Ergo Proxy XVI busy doing nothing. Now, the most obvious connection between these two anime is Geneon; however, there seems to be a more subtle reference. Earlier in the same episode, there was the following book:
This may not seem all that significant, but Green is the name of a company that worked on Background Art and Backgrounds for Ergo Proxy and The World God Only Knows, respectively. The Japanese text reads ~Takizawa Taku Kanshuu~ Yoi suimin no tori kata Warui suimin no torikata (I’m not sure on the romanization). I think it roughly means something about sleeping well or not.
While that, in itself is curious, the next scene shows this book in a box marked shobun (disposal).
I tried checking the website for Green, but it was broken! Coincidence?!