I would say look at what Facebook does, they are like digital crack.
Your site should be very useful to users and you should try to gamify things a little bit as others have said.
I would recommend building out a segmented newsletter for people. Say for instance, you get a bunch of HN users that signup. If they are interested in startups, you have a weekly or monthly email that goes out that lists books on startups, customer development, lean startups etc.
You give a small summary on each book in the newsletter and the link brings them back to the site.
I have also seen email notifications for responses to forum posts that have worked really well.
One other thing, I tried your site on a Nexus 5, and the images of the books are a little shifted and oversized. I would suggest tweaking this a little as the majority of people browse the web on their phones these days.