The case involves textbooks, but it's not really
about textbooks. Instead it's about what "made lawfully under this title" means in the context of first-sale of physical items.
Under one interpretation, if you buy a book outside the US, your ability to resell it in the US without permission depends on whether or not the book was made in the United States.
Under the other interpretation, "lawfully made under this title" means more that the making of the item itself was within the legal construct of copyright. That is, not pirated or counterfeit.
This second interpretation is what the majority opinion espouses, and the dissenting opinion aligns with the first.
This may be unsatisfactory for people concerned with software or e-book licenses, but Omega v. Costco might have come out differently if this had been decided beforehand. In that case, Omega sued Costco for reselling watches because they registered a copyright* for a logo on the back of the watch. Omega claimed that Costco had no first-sale protection from copyright since the watches were not made in the US. The 9th circuit agreed, SCOTUS split 4-4 (Kagan self-recused) and the 9th circuit stood.
The majority opinion implies that it's perfectly fine to fly to a foreign country, purchase legitimately sold watches, toasters, electric kettles, and books and bring them home for sale. (Subject to applicable duties, etc.)
Yes, manufacturers will probably not just throw their hands up and give up on price discrimination. I have no idea what they will do. But the profits from Country X still might trump any loss of profit/image due to importation and resale in the US.
Scrambling chapters in textbooks might work, but then they lose some efficiency and flexibility, for example: the option of liquidating inventory to a low-margin market when a new edition comes out in the US at a higher profit margin.
* Generally brand logos are trademarked, but they can sometimes also be copyrighted. It's a little muddy, and arguments exist whether or not, say, Mickey Mouse could become/remain a trademark after the copyright protection expires.
EDIT: To clarify what I meant by "I have no idea what they will do": I am not entirely without ideas of approaches companies could take; the suggestions in other reply threads here are definitely candidates. I just don't think I can predict the specific action[s]. Maybe 'all of the above' for some, maybe the status quo for others.