I just loved this Atlas Obscura story about old ladies and the tiny, tidy defacing of library books.

Immediately after learning about the marked 7s, Grainger says, her mind started to race through the wildest of possibilities. “I’ve got a bit of an overactive imagination, so I started coming up with all sorts of theories,” she says. “Spy rings, secret romances, serial killers, the usual!” She began checking other books for the mysterious markings. Most didn’t have it, but many in a similar genre did. These, Grainger says, are “wee old women” books—often romances set in wartime Britain, which are particularly popular with older patrons at the library. “They’re quite soft, gentle romances,” she says.

These readers made a tiny mark in each library book they read, like an initial or a circled page number, to identify which ones they’ve read before. I particularly liked that these were genre fiction readers, who might be reading more for book vibes than a unique plot. As other domestic suspense readers know, all the titles involving girl and secret start to blur together (even with the occasional missing, husband or marriage to add variety), and I’m sure that’s true for other genres.

Source: The Secret Codes Hidden in the Books of a Scottish Library – Atlas Obscura