> We’re not revealing the titles of the books included in the shipment we tracked, but they are rare, meaning there are not many copies of them in circulation. Sometimes that’s because not many copies of them were ever printed, and sometimes because they are in a foreign language not many people speak.

Not even the title of one of those rare books?

> The seller agreed to put an Apple AirTag provided by 404 Media in one of the books included in this order so we could see where the book was going. And by extension, which company, AI or otherwise, was behind this massive order.

It would seem the redaction of the rare titles is a way to avoid de-anonymization and subsequent harm to the business of the seller who agreed to place a tracker in one of the books. That being said, maybe they could have chosen a better methodology which would have allowed the disclosure of the title, although ultimately I’m not sure the title matters too much outside of their claim they were “rare”.

As I understand it, "rare" in this context could mean anything—even a washing machine manual from the 1980s...

It doesn't really matter. The article suggests that they are selecting and tracking books by ISBN, which means: books that have been published or at least reprinted in the last 50 years or so. And they are trying to get as much of that set as they can, regardless of quantity, quality, or any other consideration. Which means that older books printed before ISBNs became common may be relatively safe, at least from Amazon. And some books just don't come up on the used market very often.

(small historical irony: when Amazon first started selling books, they used the Books in Print database, which included a lot of books not actually in print.)