I just finished a pretty great book called viral loop.
We all want our product to have a viral loop and not have to pay any acquisition costs for getting any new customers. I liked this because it was a history lesson about which elements worked best to grow a large audience naturally. The case studies include tupperware, hot-or-not, ebay and facebook. It went into the viral coefficient, the levers that you can control to make your product viral and which products are more viral. Recommended!