If your target market includes people that prefer physical books, make page turning much easier - or more obvious. You also don't present any good use cases -

If your target market includes people that prefer physical books, make page turning much easier - or more obvious. You also don't present any good use cases - why would I want to spend $99 on this? What does it do for me that a plain website cannot? Consider presenting the benefits of your idea using your idea, like you did with the API docs, to demonstrate a practical use case. Apart from those things - great idea!