I visited my favourite Vinçon the other day, the shopping Mecca of things for the home. I found the invisible bookshelf, and it costs only 13 €! Here are a few pictures of what the book shelf looks like in action:
Here you can see how it works:
In other words very easy, and very cheap! Vinçon you can find on Passeig de Gracia 96, 08008 Barcelona.