What makes a good book to you?

[redsfandan]

What makes a character compelling to you all? He he/she always good and is noble or can a flawed character be more intriguing? Can the character be a complete jerk and you would still find him/her engaging?

EDIT: The reason why I'm asking that question is because the main character I'm formulating is the kind of character that is torn between doing what's right and doing the pragmatic thing. He has to do things that are unpleasant in a leadership role. However, I don't want to make him so cruel at times that he's unlikable to the reader. Where is that fine line at?

IslandRed already touched on this but I also think backstory can be very important.

Why does the main character do what they do? What motivates them? What makes them the kind of person they are? Including 1-2 things from their past can make it easier for the reader to relate to the kind of person they are and maintain interest.