They have all the information that it could possibly be useful to know about a prospective partner. How much do they spend on books? Which books do they buy? How close to publication date do they buy the important ones? Their reviews record would probably be handy too, just to make sure they understood what they were reading. Amazon has locational data for all its customers of course, so all they'd need to do is tweak some privacy settings, add a photo upload facility and let people search for cute potential partners near them who love the same books.
Lots of companies have real, verifiable data about their customers (and Amazon happens to have the sort of data that I would consider important). Screw letting online daters select the stuff they want to show onthe basis of their own wild guesses - a dating service that just laid out all the data about its members and let people decide which bits of those data they cared about would be much more interesting and useful.