Sweltering weather aside, it’s already the first week of school at thousands of colleges and universities across the country. For many students, that means “shopping” for classes—i.e., they’ve signed up for more than they can take, and plan to drop what they don’t like after the first meeting. A quick perusal of the #RealReasonStudentsDrop hashtag offers a plethora of grounds for early-semester dismissal, some of which are legitimate and heartbreaking, and some of which are ridiculous: finding out “the World Religions course wasn’t ‘why other religions are wrong’ ”; discovering that the college uses plagiarism detection software; not realizing an online course would be “this much work.” But there’s one red flag on a syllabus that should send any savvy student running for the registrar: If your professor requires you to buy his or her own books as course textbooks at full sticker price, get out now.

Heed this simple warning, and you are almost certain to avoid your institution’s most pompous, self-serving twits, the real-life versions of Gilderoy Lockhart, the preening heartthrob fraud from the second Harry Potter book. (He was played with raucous smugness in the film by Kenneth Branagh.) Alas, the cash-strapped Weasleys almost go bankrupt buying Holidays With Hags, Gadding With Ghouls, and five of Lockhart’s other hyperbolic memoirs. Students, what you’ve always suspected is true: The Potter universe is real, and directly transferrable to the college experience—or, at any rate, at least Lockharting is. So common and inexcusable is the practice that both the New York Times’ Ethicist and the American Association of University Professors have weighed in. “[T]his should be avoided as much as humanly possible,” says the Times’ Chuck Klosterman. “[S]tudents in a classroom can be a captive audience if they must purchase an assigned text that is not available either on library reserve or on a restricted website,” explains the AAUP.

Especially in the so-called soft disciplines (the humanities and social sciences), assigning one’s own work is an eye-roll-inducing ego stroke. (There are exceptions in the sciences, and I’ll get to them shortly). But it’s not the egomania of it that’s most galling. After all, many academics are by nature egomaniacs, because simply to muster up the discipline to write a 300-page monograph on some difficult and arcane subtopic means that you must find your own theories about that subtopic monumentally interesting and important, and that’s how knowledge gets produced, so, great. What’s infuriating (and, I’d even say, unethical) is own-work-assigners who actually make students pay full price, rather than just sending around a PDF of uncorrected proofs. This is gaming a system in which they are already among the very few winners.

For any professor who is in the position to assign his own books (plural) is almost certainly tenured. The standard salary breakdown for a tenured or tenure-track professor is: 40 percent for teaching (instructional time, course prep, course development, office hours, grading, etc.); 20 percent for “service to the university” (committee work); and, finally, a full 40 percent for research, i.e., writing the aforementioned arcane books. So this means that your professor has already been paid to write that book she’s making you buy. And this is indeed why, to this day, the “advance” for almost all academic books is zero dollars. (By that rationale, I suppose I’d like to see some adjuncts—many of whom have written books in the futile hopes of making it onto the tenure track—assign their own books, since it’s the only time they’ll ever be paid for writing them.)

But financial opportunism aside, there is another reason assigning one’s own book for a humanities or social science course is more than just tacky. Most such books are simply so hyperspecialized, and written in oft-impenetrable jargon, that forcing a captive audience of undergrads to read them (and, presumably, pretend to like them) is both cruel and a waste of time. Those poor kids will already get an entire semester of this professor’s opinions. They don’t need to pay to read them, too.

There are important exceptions to this warning, however. If, for example, you are taking a mathematics or science course, it is entirely possible that your professor literally wrote the book on what you’re studying. And thus, though I still find it rather gauche to line the pockets of extortionate textbook publishers by unloading $300 for a book whose PDF the author could easily (albeit quasi-illegally) send you for free, in this situation it could be warranted.

Another important exception is the professor who charges, say, $20 (or, better yet, nothing), for a bound photocopy or PDF of an unpublished textbook or a manuscript-in-progress. Again, especially in math and science, many dedicated instructors have simply not found a text that satisfies them—the material is in a wonky order; it’s so abhorrently written it causes more confusion than it solves; there are too many or too few problems and examples—and thus the profs fighting with the textbooks constitutes way too much of the class. So, they simply write their own texts, tailored to a particular course. This is actually often the mark of a good instructor.

Aside from these important exceptions, however, the practice of tenured professors assigning their own books is usually damaging to students, because it inflicts both uncouth financial practices and egomania upon a population that often doesn’t know any better. But now you do! In most situations, I’d steer clear of any humanities or social science professor who assigned, without a substantial (preferably 100 percent) discount, even one of his own books. Take it from magical me: This person is almost certainly a raging narcissist—and furthermore, just the kind of person who might, like Lockhart did to Harry in the Chamber of Secrets, ditch you just when you need his guidance the most.