If you push horizontally on your book with a force of 1 N to make the book slide at constant velocity, how much is the force of friction on the book?

In order for the Fnet force acting on the object in this case book moving at constant velocity, it would need to be equal to 0, as there is no change in acceleration and or direction occurring to the book, as you are pushing it at constant velocity. Thus to make Fnet = 0, the value of the force of friction must also be 0, as by subtracting Fapp - Ffriction = 0.