I've been a published author since 2001, but have always had to rely on my publisher's semiannual royalty statements for some clue of how well my books were selling...or obsess over Amazon.com rankings and try to extrapolate how many book sales a ranking of, say, 49,322 might mean in hard numbers.
So this is big news, and very cool for authors like myself: Amazon.com has made Nielsen BookScan data available free of charge to its Author Central members.
This means that I can track how well my books are selling, in close to real time, and also get an idea of how my marketing efforts are paying off. Are the Nielsen numbers one more thing to obsess over? Sure...but as an author, only part of my job is writing books. The more time-consuming part is selling them. I see this data as a tool I can use to sell my books more efficiently, and hopefully sell more of them, too.