Here’s a very interesting post about Kindle ebook sales from the Foner Books site. The author has a set of graphs that you should check out. Here is one of his conclusions:
… Another interesting thing is that the sales curve suggests (in the reliable section, no less) that Amazon is selling north of 600,000 Kindle eBooks a week. If we stick with the guess of around 600,000 Kindle readers in existence, that would mean that the average Kindle owner buys a book a week. That may be possible, or it could be that a lot of iPhone users are buying Kindle ebooks. If we double our estimate of the number of Kindles sold to 1.2 million, that would lower the Kindle eBook purchase rate to 2 per month per Kindle owner. Insider estimates for the number of Kindles sold are all over the lot, from around 400,000 to 1.5 million, my own guess is that when Amazon has actually sold a million Kindle readers, they will announce it publicly.