This is a tough question to answer; in fact, I am not sure that there is an answer, as this is subjective to the book matter and the reader.

The success of a book is to me the following –

1. Unable to put down, and begrudging putting the book down if I have too!
2. A book where the storyline remains with me long after I have finished it
3. A book that I want to tell everyone about
4. A book where the storyline can either parallel something in my own life or that I can identify with or perhaps that I want to explore further.