It's possible that the ad is appearing in a CSS space that is not wide enough to show the "Ads by Google" disclaimer. This could happen for two reasons: 1) the font size causes the ad to be bigger than the available space (if its set to pixel size); 2) the web designer did not allow for enough pixels in the CSS.

I didn't see it – then I remembered to turn off AdBlock ;-)

Clicking further to the book preview shows a similar box on the bottom, but this time with proper "Sponsored Links" label. I think it's just an accidental ommission on the main results page.

books.google.com/books?id=YCtT ...