This exact thing happened to me and for weeks it would spontaneously cross my mind. I thought one of my kids did it yet I couldn't guess why, until one day it hit me: they were on vacation with their aunt when I received the books and found the toilet paper, and the book was slightly damaged when I got it. Then it dawned on me that someone had read this book before, likely while on their toilet.

For that and a few other reasons, I don't get books from Amazon anymore.