Have you purchased a college course required book recently?
There is a market monopoly by Pearson, Wiley,Cengage, and McGraw.
Buy the eBook, or the actual book with a CD in the back, but cannot access the pictures because the code can be use only once! (often the codes do not work at all)
Updated every 2 to 3 years, minor changes sufficient enough the break the previous versions. e.g., randomized tests, samples and alike.
Captive audience. If Jacky teaches the course, bet your bippy it is Jacky's book you will be buying, no ifs or buts about it.
I can do the same for certification. Have you seen the PMP certification book? Grey paper with gray text republished annually, meaning of words and descriptions are changes and tests are adjusted specifically to confuse on wording. Or, have you tried to by an international standard like ISO? $300 spiral binder, assigned to you, cannot be transferred.
So, are books not too expensive? Depends on the type of book.