First, if you’ll create yourself an account, A9 will remember you previous search queries. Nothing new here, though. While most of search engines don’t do it, most of the browsers do.
Secondly, it is possible to see “Site info” for each search result, where some specific information can be found, like contacts, description, etc. The weak side of it is that this information must be submitted by a human and is not generated automatically.
It is also boasted that A9 has some “Search Inside the Book” ™ technology which you can use to find text in Amazon books. I didn’t have any piece of text in mind at the moment, so I couldn’t test if it works or not.
The last nice thing I’ve noticed about this search engine is that it is very easy to operate by just using the address bar of the browser. That means that direct queries are easily constracted. For example: http://a9.com/leonid%20mamchenkov.
People who work with Amazon.com a lot will probably find this search engine handy. Me – I don’t like the peachy colors. :)