The force in newtons required to rip the phone books apart cannot be calculated without the force given in pound-force. The friction between phone books is high due to large contact surface and the interlocking of paper fibers.
The conversion factor from pound-force to newtons is given as 1 lbf = 4.44 N. However, to calculate the amount of force it took to rip the phone books apart in newtons, we would need to know the force in pound-force (lbf). Without that information, the calculation cannot be completed.
The friction between the phone books is high due to the large surface area of contact and the interlocking of the many paper fibers. When the pages of two phone books are interleaved, they create a significant amount of surface area and tiny 'hooks' at the edges of the paper, which increase the frictional force necessary to pull them apart.