F I pinch hold a book, with 6N on either side

[dgamma3]

If I pinch hold a book, with 6N on either side why is there only one force, of 6 Newton on the free body diagram?

for example, the book has this free body diagram. http://i.imgur.com/pwtFZB6.png

there are two forces applied to each side of the book. so why is it, than in the free body diagram, there is only 1 force (of 6N). shouldn't you have two forces pointing away from one another, one with 6N (pointing right) and the other with -6N (pointing left), which would cancel out?

Edit: this is the explanation in the book.
Consider the free-body diagram below. The force of the fingers on the book is the reaction
force to the normal force of the book on the fingers, so is exactly equal and opposite the normal force on the fingers.

thanks
daniel

 

[haruspex]

I would guess that the F in the right hand picture should be an n.