Or you put them in a sealed environment with no oxygen, killing every single one of these beetles.
I'm not sure that the more lethal option is "tame".
https://www.reddit.com/r/IsItBullshit/comments/orpifq/isitbu...
Also there exist "ultra low" freezers which can bring temperature waay lower than the regular -20 Celsius. Like -80 or something. I doubt any bug or egg can survive such environment, although the books should suffer no harm.
None of these are concern with the hypoxic treatment they choose. Plus the nitrogen atmosphere treatment is so much simpler on the practical level. Instead of bringing in freezers and powering them for the whole duration of the treatment all you need is some crates, plastic bags and nitrogen bottles. Makes it much easier to bring the treatment where the books are, thus you avoid all kind of complications with transporting the books.
The hypoxic approach needs to last at least until eggs hatch, otherwise you're back to square one. And I'm not so sure if a plastic bag can hold tight for long without leaking (nitrogen out, air in).
Also, although I assume this is a very rare ability among insects and probably not applicable to the "drugstore beetle" from this article, check out this insane fly species I found while looking for freeze tolerant insects: https://en.wikipedia.org/wiki/Polypedilum_vanderplanki It (or its larvae, anyway) can survive temperatures as low as 3K!
They don't have to treat them all in the same place. They could use more than one freezer.
Insecticides will damage the natural fibers. The risk is that they damage the books more than the beetles would.
Insecticide or desiccants directly on the books, for example the natural adhesives, could cause the adhesive to crack, destroying the book.
I wish I could do this sealed nitrogen process. At the moment, it's spraying cedar wood with lavender and sticking into the less accessible places where the beetles are likely burrowing, and vaccuuming regularly.