Subscribe to Book Price Changes at

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, 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 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 For instance, to monitor my book Take Control of Your AirPort Network, you would type the URL into a news aggregator or even (in intelligent aggregators) just which would allow the aggregator to find the XML-based RSS feed on its own.