The correct reason why the textbook feels heavier than the pencil is that the textbook has more mass than the pencil, which results in a greater gravitational force acting on it. The gravitational attraction to Earth is proportional to mass, so the heavier object (the textbook) will exert a stronger pull and therefore feel heavier when you hold both objects.
In simpler terms, the reason is that the textbook has more mass than the pencil.