Ideally, what should the stamp say?

It says in the OP's link. I was just wondering if book buyers respected the stamp and furthermore if the people stealing the books recognize it enough to be deterred.

Yes the places that buy used books respect the stamp. They usually take all the books and take them to some place Half Priced Books https://www.hpb.com/ for some quick cash.

Places like that won't buy them if they are stamped.

They could try selling them on Amazon or E-Bay, but that's too much effort for quick cash to get a fix.

Thanks very much for the info. I'm building one now in a city with a fairly rough drug problem and I had no idea the stamp existed.