Ideally, you'd use HTTP Content-Negotiation to support retrieving the ISBN URL with no specific extension, so that the webserver can return the most appropriate format for the client accessing it.

Sadly it's not out in the UK yet; Amazon.co.uk is currently reporting a ship date of Sept 2008 - which I'm hoping is wrong?

(Though you can pick up a hardback copy imported from the US for about £13.)