What is average mass of a book?

The mass of a textbook, or anything else on Earth is equal to its weight. Take it to the Moon, for instance, and the mass of the textbook equals its weight on Earth. Take it to planet Pluto, and the mass of the textbook is still equal to its weight on Earth. Take it anywhere, and its mass is still equal to its weight on Earth.

Most large textbooks will weigh a kilogram or so, though some can certainly weigh more or less than this amount.