Google added some results for "harry potter book 6", and placed these 3 results between lines (like other search engines do with their sponsored listings) and after it added more results for my original query. What happens is that Google gave me some results for what it considered to be more relevant. So it is a suggestion, just like the one you receive when you misspell a word and get the "Did you mean" line. I would have a suggestion for them too, not to place these results in the middle of the search but at the end of the first page of results, and one result would be enough. Why? Because other search engines place ads like this, so Google would only create confusion with this.
Matt Cutts (famous Googler :) ) has a very interesting post on his blog regarding a search on "to be or not to be" for the same subject.
Another feature I observed in some Google results. Try this search on book.
2 things in here. First is the fact that Google automatically added some of the menus for the barnesandnoble.com website (highlighted "Home - Books - Used & Out Of Print - DVD & Video" part). I've seen this for other searches too, but I'm sure it is still in a test phase.
The second thing would be a strange observation. Did you see how the book ad result for aol.com looks (highlighted on the right side)? Just a simple observation.
Other links:
Matt Cutts: Gadgets, Google, and SEO
Google Suggest
Another Google UI Test?
Google UI Test are Not Commercial Ads
