I read the easist way to do this is to ask yourself. Would you pick up this book? if Yes then:
1. What caught your eye
2. Did the synop cause you to want to buy it?
3. Why does it appeal to you?
a. genre?
b. Cover?
c. Age?
d. reason you read? escape, educational? mental challenge (discover who did it?) to be scared?

These are your demographics.
My readers are 30-90 yr old men and women. Most are curious about the plot. they want to find the truth. They want to be entertained and turning pages. If I can't accomplish that I've failed them. They have higher expectations since my first book is quite good. Here is one clue. They read my book because some one referred them. I don't know about the e-version so much unless they post a review. Some just tell me or my friends they read it. I have to meet those expectations.
I have had 14 yr old boys and 16 yr old girls read it. One girl read the first few chapters and put it down. Later she had to read a book for school and picked it up again. her mother told me she fell asleep reading it and read it while brushing her teeth in the morning. She told her mom she really liked the book and told her teacher about it...
I know I can sell at least 150 books quickly... if they don't like it then the other 200 won't buy it... Its all about word of mouth.

          Tina