I got a rather mysterious phone call from one of the bookshops which sells my book. The owner said that she'd sold one, and the other was stained, but I had to come and see what stained it. "It's a surprise. And I must say, the customer's attitude surprised me."
Obviously that roused my curiosity, and I went.
My best guess was that the stain was from something like an insect that snuck in there and then got squished, and that the customer had been snooty about it.
The stain was actually quite faint, although I would be reluctant to sell it like that. Which is fine - I want a copy for myself anyway. But the real surprise is that it was from a 10€ bank note.
Yes, he handed the note over to the shop keeper. And she gave it to me.
This surprises me less that it would in many other places, but it's still very nice indeed.
So thank you, mystery customer and thank you Dimaro libería in San Pedro!