Data is entirely a collection of externalities; it can collect and sort millions of user preferences and similarities, but it can never move beyond the what to the why. Data has no imagination. When it comes to book recommendations, attempts to sort or streamline or mathematize them necessarily dehumanize the process. The very nature of the endeavor, much like digesting Ulysses, requires an infinitely more complex machine: the human brain.
One example from the article I liked was the concept of liking books about cold places. This is not something you can program a computer to do, at least not currently. And even if you could, wouldn't you be saving yourself a step (and maybe putting a little more utility back into people's lives) by asking the well-read guy next to you?
I'm ambivalent, shilling for skeptical, about the way we let technology eclipse human behavior. Some of this has been for the better: do we really need people performing backbreaking and dangerous labor? But when we use it to replace the one thing we pride ourselves on so much — our brains — then I get edgy.
For the record, I find the way Amazon and Goodreads do recommendations to be useful, but only up to a point. Part of the pleasure of going to a bookstore is wandering around and having happy accidents, or discovering something that I would never have had recommended to me algorithmically. I'm not thrilled about being herded by inches into a world where we have one less fun thing to do because it's been allegedly made irrelevant.