Does it make sense to say that a textbook is about 99.9 percent empty space?

Yes, it does make sense to say that a textbook is about 99.9 percent empty space. This is because atoms are mostly empty space. The nucleus of an atom is very small compared to the size of the atom itself. The electrons orbit the nucleus at a great distance from it. As a result, most of the volume of an atom is empty space.

When you look at a textbook, you are seeing the atoms that make up the paper and the ink. However, you are not seeing all of the empty space that is between the atoms. If you could somehow remove all of the empty space from a textbook, you would be left with a very small amount of matter. This is why it is said that a textbook is about 99.9 percent empty space.