Over the years I've run across a variety of things used as bookmarks and left in Oz books. There have been assorted dried flowers and four leaf clovers, as well as popsicle sticks and toothpicks. On the right I'm showing a Christmas tag left in a first edition copy of The Tin Woodman of Oz, presented to Dorothy (how appropriate!) from Florence and Helen.
Advertising bookmarks seem to have been popular. This first one is for Stiefel's Medicinal Soaps, which supposedly possessed healing and antiseptic properties. It was left in a copy of Rinkitink in Oz.
The horseshoe design is from Bunte Brothers, and is very vague as far as what was being sold - other than it being "of superior quality and free from adulteration". This was found in a copy of The Road to Oz.
This third example is of a more recent vintage, for Corklyn Papeteries. I don't think I've run across the word "papeteries" before! I've forgotten where this one came from.
Another fun find was the original receipt from the purchase of a copy of The Silver Princess in Oz, dated May 17, 1938. This was left in the book together with flyers for two other books - not Oz stories.
I think my favorite is this drawing of a witch, found in a copy of The Wizard of Oz. Someone was being creative with triangles!
And of course there is the map of Oz which was laid into Oz books sold in 1920. I've run across a few of these, still in their books!