We don't care about the mass of book-B, the one on the bottom. Book-A is resting on something, and it's not accelerating up or down. So the vertical forces on it are balanced. The upward "reaction" force from whatever it's resting on is equal to it's own weight. Book-A's weight is (mass)x(gravity)=(2kg)x(9.8m/S2) = 19.8 newtons. That's also the reaction force acting upward on its underside.