To calculate the net force on the book, we need to subtract the force of friction from the force applied by the student.

Given force applied by the student, F = 6 N

Force of friction, f = 4 N

Net force, Fnet = F - f

Fnet = 6 N - 4 N

Fnet = 2 N

Therefore, the net force on the book is 2 N.