Google doesn't charge anything for the link. It charges when someone clicks, and it charges on an auction basis. Ad buyers set their maximum price and get placement based on that, then pay based on what competitors for the term are willing to pay.
For a search term with so little traffic, Allibris probably paid something like a penny, and certainly less than a nickel, when I clicked just now. They get not just the chance to sell a book, but brand awareness (to which they're indebted also to you, Jim).
Google has open APIs so Allibris can just port their database of titles over to be search terms on which they buy paid links. That means there's little cost to setting up the ads.

I haven't yet done that with my advertising for WashingtonWatch.com so I laboriously create a new ad for each bill on the site. Go ahead and search "H.R. 5534" to see yet another way that your tax dollars are being spent.