One get's so accustomed to searches working that the disappointment when it does not work is quite heavy.
What I was looking for and couldn't find was any indication as to whether amazon's API works for amazon.de also. Doing a site specific search for web service API at the worlds largets bookstore was less than helpful however (and you are complete right also: I should have searched for some german description but there you have it). All of the returned pages have similar rank and they about books on web services instead of Amazon's own offering.
Interestingly - the amazon.com description of webservices works nicely.