The US publishing industry is $28B. technical/educational books are 50% of that, And Google could only get 10% for that market(many other channels), and let's say they'll get 10% affiliate fee. that equals 28B * 0.5 * 0.1 * 0.1 = $140 million/year , for what they do currently.
Books are quite expensive, so adding visual links to the main search engine, would say double that - $280 million/yr. Not much for Google.
But on the negative side, adding visual links will distract people from surfing to sites filled with ads,clicking search ads, etc - so maybe it will cost Google money. Maybe a lot of money. That could be the logic behind why Google Forums, clearly a valuable service, was dumped[1].
So Google probably needs a more serious way to monetize it to justify the effort.
I'm sure Google could work out some affiliate fee/scheme for directing the user to a marketplace to buy the book whose image is shown. But if Google directs the user to it's own marketplace, it would surely get a cut, no?