There are numerous enemies of books : from fire to autodafé. Another enemy is water or humidity… if your books start to smell moldy. That's not good a sign. A little bit worry about that, I installed a small sensor in one of my bookshelf. The purpose is to monitor humidity and see if the relative humidity level is going to the famous 75% limit. I'm using an USB-based humidity and temperature sensor made by Raphaël Assénat. The software and the hardware design is free software/hardware but you can purchase from him the assembled version (maybe a good way to help the initiative). You can see on the today's graph that the humidity increased when it was very rainy outside and a window was opened. But nothing critical for the books. I'm just wondering when Ikea will include by default a humidity sensor in each Billy bookshelf ?