Amazon is now providing authors enrolled in their 'Author Central' programme access to a small window of Nielsen BookScan sales data for their own books.
The programme shows the most recent four weeks of sales as tracked by Nielsen, on a week-by-week basis. It also provides geographical breakdowns.
I've been on my Author Central Account at Amazon.com and have found the information easy to access and understand. It does not cover all books however and neither does it cover e books sales (yet), but it's a start. It's always useful to have sales information and the more detailed the better because it aids marketing programmes.
I'm hoping that amazon.co.uk where my books are also sold will soon be providing the same service.