When a book slides down on the desk, a frictional force acts between the lower surface of the book that is in contact with the desk and the upper surface of the desk that is in contact with the book. That means the frictional force comes into play when the minute roughness on the surface of the book gets interlocked with the minute roughness on the surface of the desk.
The direction of the frictional force on the book is opposite to the direction of its motion of the book. As the book is sliding downwards so the frictional force acts in the upward direction.
It acts as shown in the diagram below :-