[alexhutcheson]

Textbook publishers release new editions, at which point they stop selling the old editions (so university bookstores can't easily stock old editions). Instructors then pretty much have to assign new editions.

Students looking to save money can buy old editions or international editions online, but then have to deal with page numbers, chapter order, and exercises being different than the book used by the rest of the class.

Frustratingly, publishers do this even for topics like calculus, which has not changed a lot since the last edition of the textbook!