I got a bee in my bonnet, or perhaps a bug in my ear, about a week ago and finished the programming this evening on a system that allows you to use an RSS news aggregator to "subscribe" to updates of the price of a book at a number of online bookstores. I run isbn.nu, a book price comparison site, but you need to visit the site to see the price of a given book at about 16 bookstores, mostly in the US.
I've long thought about adding a feature that would allow people to sign up for notifications about when a book price changed or became available at a given bookstore. Perhaps you were waiting for a used copy of the book, or wanted a book to drop below a certain price. I may still offer email notification, but it struck me that I could immediately provide an RSS 2.0 feed with very very little effort.
In RSS, a news aggregator retrieves a page on a regular schedule to see if any changes have occurred. If so, it displays these changes, typically in the form of a headline or a line item in a list. I have provided each book at isbn.nu its own unique feed that's generated each time a news aggregator queries it. Whenever the price gets stale--typically every 24 hours--it retrieves a new price from a given bookstore. This price shows up as a new item to the RSS aggregator, which then displays it.
It's a little hack that should work quite well for people monitoring titles. I've added the code so that the RSS feed shows up automatically within Firefox and other RSS-capable Web browsers. I'll be adding more cues to people visiting the site shortly.
The pattern, by the way is http://isbn.nu/ISBN.xml. For instance, to monitor my book Take Control of Your AirPort Network, you would type the URL http://isbn.nu/0321321162.xml into a news aggregator or even (in intelligent aggregators) just http://isbn.nu/0321321162 which would allow the aggregator to find the XML-based RSS feed on its own.