So everyone's clear, this service Alikewise does actually have a link that just says "recommend a book to this person." It would probably be a good place to do a one-shot ad hoc analysis of members' book preferences and recommend the right books to the people you consider most likely to appreciate it, and maybe return every year or so or more if userbase growth spikes; but OkCupid and other non-dating sites almost certainly have more relevant data, if only you had a socially acceptable mechanism of suggesting books and doing nothing else bad, unlike fooling people into thinking you're looking for a mate or sending unsolicited messages on social media platforms. Not elegant solutions. As is you could only use the other data to possibly refine your estimates of Alikewise's users' preferences, and probably not much else.