I bought a pair of prop pole of anti-earthquake shelf holder at Seiyu. It prevents that a shelf falls down to the floor when an earthquake comes. It cost 1100 yen ($11.00 €8.46) for 2 poles. I set up props between my bookshelf and the celling. There was more expensive version in the store. It was about 2500 yen for one pole. My prop is partially made by plastic. But in case of the expensive one, all of parts are made by metal.