* The book is at rest. This means it's not accelerating.

* Newton's First Law of Motion: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Therefore, since the book is at rest, the net force acting on it is zero.

Here's why this is important:

* Gravity: There's a force of gravity pulling the book downwards.

* Normal Force: The desk exerts an upward force on the book to prevent it from falling through.

These two forces are equal in magnitude and opposite in direction, resulting in a net force of zero.

So, the answer is that the book is not exerting any force on the desk. The desk is exerting a force on the book, which is equal in magnitude and opposite in direction to the force of gravity acting on the book.