Although I didn't have €50 to spend on the book I found, The Invisible Wall by Stephan Kaluza, his website provides images as well as essays by Heinz-Norbert Jocks and David Galloway from the book.
There is also an interview online, with scenes of the photographer on location (in German) by Institut für Kunstdokumentation.
Galloway, D. (2009) Walking the Line. [Online]
Institut für Kunstdokumentation  Stephan Kaluza - project Berlin Wall. [Online] http://vimeo.com/6414799 [18/04/2010]
Jocks, H.-N. (2009) The Spirit of the Absent. [Online]