Bookmarks that are book friendly are made of acid-free paper or something very thin. You are looking for bookmarks that won’t stain, crease, tear, or otherwise cause stress to your pages or binding. We have been horrified by what we’ve seen in the last year or so that are being sold as bookmarks. Pieces of pierced or cast metal, clips that are supposed to fit benignly on the edge of the page, woven tapestries with who-knows-what chemicals in them, and some with elastic bands. These could put irreparable wrinkles, holes, tears, or discolor your pages.
What to look for in book friendly bookmarks
The best is simply a strip of acid-free paper, and that does not include Post-its. The adhesive on Post-its is not only strong enough to lift the text from the page, but sometimes will also lift a layer of the paper itself, especially older delicate paper. So if you need to use a Post-it as a temporary bookmark, be sure to put it on the margins of the page and not over the text. If you so use a Post-it, carefully remove it from the page. Check to see if any of the adhesive was left behind to either discolor the page or pick up dirt.
Dogearing the page can cause the corner to fall off especially if the page is older delicate paper or acidic/brittle.
Notice we said acid-free paper and not cardboard, even though you can get acid-free board. The thickness could stress the binding of the book especially if you need more than one bookmark. Placing the bookmarks closer to the fore edge of the book rather than the spine will help from straining the binding.