Note the Google Books onebox may appear together with other oneboxes, “universal search" style. A search for Daniel Clowes for instance shows the image onebox on top followed by the books onebox. As is the case with other oneboxes, these appear in web search not just when there’s a hit, but only when the hit is considered relevant enough too**. E.g. comic book artist Ivan Brunetti is also an author within the Google Books database, but a search for him in Google web search will not bring up the books onebox.
*The image URL for the icon is books.google.com/books?id= 26iuAAAACAAJ&dq=&printsec=frontcover&img=1&zoom=5
You can also change the domain to www.google.com, and the image is still served. And you can change the zoom parameter to 1 to see a larger thumbnail. The “26iuAAAACAAJ" value is the book ID, so we can access the book detail page by heading over to the URL google.com/books?id=26iuAAAACAAJ
**I once asked Google how they determine relevancy for onebox display, but they only provided a bit of a non-answer: “We have algorithms that determine when we think showing an image would be useful to our users. As with everything else, these algorithms are evolving, and we are continuously working to improve this experience.”
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