How to promote my website/app to independent authors? where are they? how to talk to them?

[S.K.]

I think any author who signs up for your app is a fool. You clearly have no idea how to offer the writer anything she can't already get for free without you. What possible advantages would the author get if she does this? From your question, I see only negatives--no idea about the publishing industry, no plans on marketing, and no doubt the rights would be tied up while the book just sits there, available but not selling because your marketing agency doesn't know what it's doing.