R. Preston McAfee, a Cal Tech economics professor, is annoyed at how overpriced textbooks are. “‘The person who pays for the book, the parent or the student, doesn’t choose it,’ he said. ‘There is this sort of creep. It’s always O.K. to add $5.’” To fight back, he’s foregone the potential six-figure advance traditional publishing would have granted, and published his textbook online for free.