Buying multiple copies

Just opened Gordon's secret shop, and bought 3 copies of each of the relevant cooking skill books, passing the spares to my other 2 characters via the bank. This surprised me, because I had a clear memory that on a previous occasion when I bought a skill book from an NPC, the NPC only had 1 copy of the book, so that after I bought it the secret shop tab was empty. This meant all 3 of my characters had to unlock that secret shop to get a copy of the book. Unfortunately, I can't remember for sure which NPC that was, but suspect Nora or Agnes.

So, this means either

1. The secret shop mechanics have changed, and all secret shops now have multiple copies of each item; 2. Some shops have multiple copies and some don't, in which case it would be useful if we document which ones are which; or 3. I'm getting too old and shouldn't trust my memory anymore.

I'm hoping someone can confirm one of the first 2 options.

Incidentally, after 65 ring gifts failed to open Gordon's shop, I gave up and did rafting missions for likeability potions, eventually opening the secret shop after gifting 5 potions.

If it counts, I don't remember secret shop books ever having disappeared from the shop after purchasing.

Perhaps you must have gotten it confused for something else. To my knowledge, Stewart's secret shop's scary library passes and Glewyas' normal shop's kitchen dungeon passes are the only items that are in limited stock.