Worldwide financial situation
If the worldwide economy is going very good, most of the industries will be able to claim the same thing. But if we consider the current financial situation, we notice that not only the real-estate and banking industries are the ones to suffer, but also any other industry. The side effects of a bad financial situation are: less money to invest, less consumption, less trust in institutions and organization, less production, less income etc.
One that interests us in particular is the annual income people get during a financial crisis. Of course this income will be lower that usual, but what do people do with it? Do they still consider that a university degrees still deserves the huge investment or the 10-years credit at the bank? They could easily consider that, as they had earned enough money to satisfy their needs in a job that didn't require a degree, they should start learning at home and give up on school. If the level of education decreases then there will also be less and less people to read, or their reading activity will be far lower than today. Not only the study books will have to suffer from this, but also all professional books and even the novels.
Another impact the worldwide economy can have is to make companies consider twice any investments they make. And this investments refer to R&D, new book manufacturing, new technology to generate lower costs.