I'm a bit behind Matt in sales, but I just launched a site the other day that does something quite similar to RSStalker, but specific to best selling books: http://yowzas.com

Best selling book lists are everywhere, but this one is near real-time due to AWS. I find myself browsing the site daily reading book reviews. I have category specific ones like http://business.yowzas.com and just like RSStalker, I have RSS feeds for each major category of best selling book lists.