A book rests on the shelf of a bookcase. The reaction force to the force of gravity acting on the book is

F'g = force exerted by the book on the Earth.

Explanation:

According Newton's 3rd law, that says that for any action there exists a reaction (acting on different bodies) the force that the mass of the book exerts on the mass of the Earth, is equal and opposite to the one that gravity exerts on the book, acting along the line that joins both masses (in this case the book with the center of the Earth).The magnitude of this force can be obtained just applying the Newton's Universal Law of Gravitation to both masses, being the direction directly upward from the center of the Earth.It must be NOT confused with the normal force exerted by the bookcase surface on the book, that just avoids the book to be accelerated downward.

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