Question

why book go back when we apply force but newton third law said it exerts equal force

Answer 1

The book goes back because the force applied by us is greater than that applied by book.
Further newtons first law also tells us that if unbalnced force is applied a body moves.
So here we are applying external unbalanced force which is greater than that applied by book.

Answer 2

book go in the forward direction and your hand goes in backward direction after applying force and newton laws says that there is an equal and opp. action and reaction force.so it is correct.