Amazon has 6 different international stores: Amazon.com (United States), Amazon.co.uk (United Kingdom), Amazon.ca (Canada), Amazon.fr (France), Amazon.de (Germany) and Amazon.co.jp (Japan)
Prices of the same books can differ quite a bit on these sites and also exchange rates fluctuate heavily lately. For example the euro dollar exchange rate has been changing heavily. In February 2009 the Euro was worth approximately $1.25. In November 2009 it was $1.50 (a 20% change) and in May 2010 the euro was back at $1.25. Also the British Pound has fluctuated heavily. As Amazon does not change the prices of their books often it can very well be that these exchange rates have a large influence on the final price of the book.
So depending on how different prices in different countries are set and the current exchange rate you can save money by buying the book you want to read in another Amazon shop from where you normally buy. What you can do is search for price of the book you want to buy on each of these sites add the shipping costs and do the exchange rate conversion. This can easily save you 10-20% on your purchase, but is quite cumbersome to do manually.
As with most of these cumbersome things there is a website to help you with this. In this case that is CheapRiver.com. When you search for a book you automatically get the prices on the different amazon sites and it takes into account the latest exchange and shipping rates. The resulting prices are displayed in a table with the cheapes option being highlighted.
Finally do not be afraid of ordering at an Amazon site that is not in your native language. The sites work identical and look the same as the one in your native language. Just the text is translated.