I've been poking some more at isbn.nu, the book-price comparison service I run, and with a suggestion from a news aggregator developer, have added RSS syndication to search results for books. If you search on an author, subject, or title, you can now subscribe via RSS to the results of that search. As I update the database, typically weekly, any new books show up in that feed. For instance, search on Stephen King and the XML feed in RSS 2.0 format is this.
I've embedded the RSS link in the meta tags for the page, which is a nifty way to allow Firefox and many news aggregators to recognize the RSS feed's availability automatically. Kind of slick.
To start with, I've limited the results to the 25 newest books by reported publication date. That seems to make the most sense given why someone would subscribe to a search results feed. I'll experiment over time with this to see if there are other searches that would change over time in a way that a subscription would be useful.