The book's potential energy will be converted entirely to kinetic energy at that point in the fall, since we can neglect losses due to air resistance. The potential energy is given by PE=mgh, where m=1.2kg, h=0.9m, and g=9.81m/s^2. This is the acceleration due to gravity.
This gives:
PE=1.2*0.9*9.81=10.59J
This is also the kinetic (mechanical) energy right before the landing.
