Three books are at rest, in equilibrium, on a horizontal table as shown. The weight of each book, which is equal to the force
gravity exerts in the downward direction, is given. What is the
net force on the middle book?

Net force = 0

Explanation:

Short if the middle book is not acceleration in any direction, the net force is zero.

Long refer to the attached free-body diagram (FBD) to see why the net force is zero.  Skills to draw FBD are  essential in solving problems in statics.

Top book exerts -mg (downwards) on the middle book.

Middle book exerts -mg (downwards) on the bottom book.

Total downward force is -mg-mg = -2mg

By Newton's third law, when an object is not in motion, reaction equals action of -2mg in the opposite direction, therefore the reaction (upwards) is +2mg.

This makes the net force of -2mg (downwards) + 2mg (upwards) =0

i would say