The reason demand falls is that people find textbooks less useful relative to other goods and services at the higher price. An example might be a builder needing to figure out some complex geometry for a special roof. At one price for the book, he may feel buying a textbook and studying it for a couple of evenings is the best way to get the information he needs. But at a higher price for the book, he may opt to have subcontractor do the work and spend his time doing something else.
