Ideas are cheap. It's the realization of them that makes difference.
Books are good for obtaining background knowledge on the subject. But you will have to take a pencil and a paper and write down answers to the following questions: "What value will my application bring to people? How will they benefit from it? How will it save their time, make their life easier in some way or another?"
The answers will guide the further research. If your application isn't going to satisfy a need, it's unlikely that people will pay for it. And you need to 'go to the fields' and ask people if they will like your application and be ready to pay for it. Their answers will give you a lot of new ideas.
In other words, stop thinking fancy marketing terms like 'target audience', 'segmentation' and suchlike. Think simple - 'Will people need my application?', 'Why would they pay for it that price?'.
There's a looong way from having an idea and making that idea commercially viable. You can end up selling something entirely different from what you envisioned at the start.