I recently came across a good book on marketing called "Contagious: Why Things Catch On"
Here's a video of him promoting it
https://www.youtube.com/watch?v=FN4eDk1pq6U.
It looks really good. The basic "STEPPS" are:
Social currency -- What about this will make the person telling others about you, cool?
Triggers -- What events will naturally occur in their life to remember (and want to talk about) this?
Emotional -- How will this make them feel?
Public -- How available is this to everyone to see?
Practical value -- How does this compare with all of the other ones out there?
Stories -- A short, memorable story, joke, or anecdote about this that is easy to share quickly?
They sound like good things to think about. The "this" doesn't have to be the book; it can be what ever promotional (or other) thing you want others to help you share.
(Full disclosure: I checked the book out of the library, but I haven't gotten past the first chapter yet.)