Unread Book Indicator

My wife reads 100-150 books per year, and therefore buys a metric sh*tton of books. As the collection have grown, her system of having the unread books laying around in piles and the ones she has read on the shelves has become hard to manage, so I made her these little things.

All her books are now on the shelves, but she can easily identify the ones she has not read yet.