That element of the Half-Blood Prince was taken from real life. Used textbooks have been preferred by students for precisely for this reason (well, cost too, but this is a well-known benefit) in colleges since... well, probably since textbooks have existed.
Halfway through my college experience (around 2002 or so), the university started putting up blinders in the bookstore while they stocked shelves and wouldn't let you buy your books until basically the first day of class, specifically in an attempt to stymie students finding their books, looking them up, and buying them at 1/10th the price online.
I mean, it wasn't my first experience with the university prioritizing profit over helping students, but it was definitely emblematic.
Most of us figured out that we could get along fine not having the textbook in the first couple of weeks of class. But ultimately, the university was out to actively sabotage the used textbook market. The only source of used books was online. So I never got to experience this community of used book students.
They go through some pretty extreme lengths to get you to waste money on buying books from them. The English classes my university published a new "reader textbook" ever quarter. It was just a crappy bounded letter paper book with section from various novels that they change up every quarter so you couldn't use an old one. The on campus copy center and nearby kinkos/staples/officemax/officedepot wouldn't photocopy it but a half hour drive out would reach stores that didn't care. A photocopy costed about 1/5 of the price the university was selling it at.
Years ago at the university, we had to buy straightup photocopies of articles and such out of books/magazines/whatever that the class would be taught off of at the campus bookstore at prices much higher than per-page copy.
Something about paying the source for licensing and distribution was the reason given.
On the other hand, my physics prof in relativity made us buy photocopies of his lecture notes, since he didn't like any of the available textbooks. (Don't sneer - I'm pretty sure he was better than any of them.) His notes cost, IIRC, $4 for 90-100 pages. This was 1983, but still, four cents a page is pretty good.
I had this one class on Picasso where the professor absolutely refused to use the only real book on the subject because the author didn’t properly give him credit from his thesis research so he handed out a folder of articles we passed around to make copies of. No licensing and distribution fees involved.
Some book publishers also release new versions that change nothing other than make slight changes and reorderings of the exercises so that you can't easily use an old version for your homework assignments.
I remember we had a forum group where 1 person each assignment would go to library to find out which question matched the one in the old book so his or her fellow peers all went out and bought the old one
At my school the savvy students wouldn't buy their books until after the first session of a course anyway, since the professors would often, in that first meeting, explain that some books listed in the syllabus as required were actually optional, or that they'd support some set of older editions of a book than the syllabus listed ("it lists the 5th edition, but it's OK if you get the 4th, and if all you can get ahold of is the 3rd, see me after class and we'll get something sorted out—but nothing older than that").
That's what we did. We waited for the bookstore to open the aisles, found the books, ordered them online, and waited a few weeks to get them. This was before Amazon Prime shipping, you see. The professors were usually sympathetic.
Except my political science prof. So I just dropped the course. Fuck him and his unironic bowtie.
All the students in a course I was teaching apparently were using Chegg for the previous year’s textbook so I decided to use as a supplement (to Strang [which was not the previous year’s book but had all the answers online anyway]) an old Mir publishers book on Diff Eq which I can’t remember exactly how I got (either a bookstore in the French quarter or maybe a library remainder sale).
At any rate it turns out the English printing is so rare not only can it not be found on Libgen - the few copies online are selling for hundreds of dollars (which I certainly would not have paid for it). So not only did I luck into a paper fortune (I suspect this is a rather illiquid market - plus I had to go through and fix a bunch of typos by hand, so much for the Soviet STEM educational complex) the kids _definitely_ couldn’t find this on the Internet.
I may soon - there is a book scanner in the med school library and I doubt very much they care about a Soviet copyrighted book (if USSR even asserted such a thing abroad or Russia does now for them).
Yeah, I actually bought the textbook in '03, a few years before Half-Blood Prince was published. The whole experience did ruin the twist a bit as I saw it coming a mile away.
I don't think the parent is saying that the professor annotated the book, as that probably doesn't make any sense.
As I interpreted their story, the professor had a habit of asking questions and then answering them himself, if nobody offered an answer. Students wrote down those answers in the margins of the book.
Over time, the book collected a lot of these notes from different students.
