The marketability, the success of a book, ultimately rests with whether or not people will find the concept/characters/title/cover appealing.

You can write the best book you can, and that might still not be enough. Appeal isn't something that most writers can't strive for or identify. It's something even the best agents and editors can't always identify.

Nora Roberts, Stephen King, Lee Child and George R. R. Martin write wildly different books. Their writing, plotting and styles have little or nothing in common. But they all write books and characters that readers find appealing.

M. J. Rose Quotes