How would the velocity of the book change if the applied force were equal to the sliding friction force

Yes, Sliding friction opposes the movement of the book, slowing it down. sliding That's the 'kinetic' kind ... According to Newton's second law, F=ma. That is, the bear's acceleration should be proportional to the total force acting on the bear. As the bear's velocity is constant, its acceleration is zero. Therefore, the total Force acting on the bear is zero. Thus, the friction has to be equal in magnitude and opposite in direction to the bear's weight. As W=mg, we get that its weight is 9.8*400=3,920 Newton. Thus, the friction acting on the bear is 3,920 Newton