Very versatile design. These thin bent metal shelves are called Flying Vee and are designed by Massie Office. They don't require ruining a book, they're not quite invisible, yet they have a simple, razor-sharp appearance.
If you hang the Vee inverted on your wall, you can set an open book on it and mark your page. We like how these little things can sprinkle across a wall in all kinds of angles and arrangements.