I love Forgotten Bookmarks, which offers pictures of fascinating objects stowed away inside books. First-name-only Michael, the proprietor, is a rare and antique bookseller who runs through five- to six-hundred books a day and shares the treasures between their pages.
Highlights include an honest-to-God saw blade, (did the reader consider the book sharp or in need of cutting?) handmade silk embroidery, an 1893 ad for "tundum" as a dentistry anesthetic — likely cocaine, a family crest, and Commander Spock trading cards (inside a Handbook of Tibetan Culture — an anthropologist trekkie, clearly).
Some of the most notable ones Michael doesn't post pictures of, such as the pot leaf inside a cookbook and a suicide note from the 1930s.
I've long been fascinated by the objects I find in books, which have ranged from 1960s love letters to receipts for sex toys. One of these days I'll whip it together into an essay (but clearly Michael is ahead of me).
What I love about this treasure hunt through old books is that it highlights the physicality of books. You can't find stuff like this in a Kindle. Even more than their physicality, it shows books as trustworthy repositories — some of these objects are quite precious, and whether the depositor realizes it or not, they are entrusting their object to the safety between sheaves.