Since it is on a flat table, the normal force is equal to its total weight, FN=45N. This is the force the table applies to the book. The force that is causing the 45N downward is gravity, which is FG=-45N. I say negative because the force of gravity is down. So FN and FG are equal and opposite forces. They add to zero, which according to Newton's second law, F=ma, means that if the net force is zero, 0=ma. Since m is not zero, a must be zero. In other words, the book is not accelerating, it is sitting still on the table.
