I asked about a month ago are Amazon rankings Zipfian?
Morris Rosenthal, a self-publishing author, has an interest in this subject for obvious reasons; he’s got some interesting-looking resutls for both paper books and e-books. Roughly speaking, yes; the most interesting thing I notice is that the slope of the rank vs. estimated sales curve (on a log-log scale) is higher both at the head (best-seller) and in the tail of the distribution compared to its bulk. What to make of this, I don’t know.
Chad Orzel, a physicist and author of How To Teach Physics to Your Dog took his own shot at this question a couple years ago.
God plays dice 