At least, that is what they are called. If you have some room left in your library, this seems like a fun gadget. It is a row of five antique looking books that have a funny habit. I do not know how they made it, but when someone walks by them the middle book will slide out. If I may believe the website, it will produce some spooky sounds at the same time. Click here to learn more.
Thanks to Freiko for the link, via Bookpatrol.