@Doctor Syntax - textbook sales
I am guessing you aren't from the US, where textbooks are a racket to steal money from college students. Professors get kickbacks from book companies for using textbooks in their classes, and so sometimes require more than one. They cost a ridiculous amount, and there's a new edition every year (perhaps every semester now?) to prevent their resale and reuse - they have different homework problems in them so you can't get by with older editions!
Even more money is made by the professors who write textbooks, so it is worth it for those who teach big lecture classes to write their own textbooks even if the only people who use them are their own students!
Students in the US will often spend upwards of $1000 per semester on books, the resale value of which is nearly zero when they finish the class because new editions are forthcoming which obsolete the old ones.