Amazing interview of Gary Vaynerchuk - Developed a platform on Twitter and others and was offered a seven figure advance 10 book deal with Harper Collins. I would have showed him how to publish his book himself and keep 100% of the profit for himself. (I will admit that a 7 figure advance would be hard to turn away)
He talks about the importance of building a relationship with your audience. Interactivity is key.
He calls Twitter "word of mouth on steroids". Say something meaningful and the people that care about it will spread the word.
"Treat twitter like a cocktail party." Don't make everything you say about a sales pitch.
Platforms like twitter have made it possible to develop groups of people interested in the same thing across the globe who can communicate about their interest. If you can bring content to this group as an author, you will sell books.