The books are actually held up using the "Sticklebook"! The Sticklebook's "innovative design comprises an aluminium extrusion which acts as a bracket and polypropylene combed strip that grips the cover and pages of paperback books. The shelf is screwed onto the wall which itself helps support the weight of the book and is an integral part of the design. It is totally secure and fall-proof."
It's about $28 for one, and they ship to the US. Click here to check out more info and details.
Here's some more behind the door ideas from the Feb/March 2008 issue of Marie Claire Maison.
[ image via Marie Claire Maison ]