In several of her works, Rachel Whiteread is “unwalling” architecture by casting the negative of the wall i.e. the space framed by the walls.
For House (1993), she casted an entire Victorian house in London printing all the details of the original walls in the concrete. She then destroyed them letting the concrete negative as a final result.
For the Holocaust Monument (2000) in Vienna she metaphorically reproduced this process by casting an imaginary library. The books are thus appearing as the surface of the monument and the detail precision is such that the pages are actually visible.

two last pictures: Joe Perez-Green