With an online library how are you going to "serendipitously" find new books from other domains you don't know you're interested in?

I remember when I was 14 or 15, I was going to our small-town's public library once every 2 days or so, I was reading all the SF books I could find. The thing is that the SF books shelf was just after the history books shelf, and, even though I liked history, I never would have thought of reading history books if it weren't for me passing in front of that history books shelf each and every time.