StackOverflow is using a referral system to associate each Amazon Book link with its Company Account, using htttp://rads.stackoverflow.com for this service. As was pointed out during the announcement, this should be based on the geolocation. To quote Jeff Atwood himself:
We have a clever affiliate redirect that geolocates the IP and redirects to the appropriate regional Amazon store (amazon.uk, amazon.ca, et cetera).
However, when I click the Amazon link in this answer Is splitting up a function into several inner functions an anti-pattern?, I get redirected to amazon.com, i.e. the US version of Amazon. However, I am based in Germany and other Geo-Location IP tools also show this.
So to me it seems like the geolocation is not so clever after all. As it is written in the announcement that this is an actual working feature, but appears (at least for me) not to work, I would consider this a bug.