I have had my Kindle since the first day they were sold. Since then I have bought about 8 books/month. I have about as many samples “on-board” that I will buy as I work my way through the backlog.

More books per Kindle/month would – given all your other assumptions – mean fewer Kindle units in the wild than your estimate of 50K.

My guess is that folks that bought the Kindle are, given the price of the device, more likely to read/buy far more books than the average person. On the other hand, given the price of the device maybe most folks are just loading stuff from Project Gutenberg.

I don’t personally know anyone else with a Kindle. Perhaps there has been some discussion amongst the Kindle users in the forums on amazon that might give some more data as to the number of books other users are buying each month.

Amazon reported 2.4 billion in “media” sales in the first quarter of 2008. Unfortunately that includes books as well as DVDs and music. Hard to find figures that break down the percentages of each type of media. Let’s call books half. $1.2 billion per quarter, or $400 million per month.

So now, absolute cap: 6% of that is 24 million per month. That is the absolute largest possible figure for kindle content sales per month. But that would be true only if 100% of amazon’s print book sales consisted of the 100,000 titles available on kindle.

But that’s clearly way too high, since only about 100k out of 2 million book titles are in the “kindle universe”. And the Kindle Universe contains large numbers of public domain, specialty, self-published books that for all practical purposes have nearly zero sales. Only perhaps 25,000 of those kindle titles are major publisher mainstream trade titles that have any significant sales. We can conclude that Kindle is not “top skimming” from the long tail of Amazon’s books, but rather the universe of kindle titles has it’s own long tail. Our best guess would be those tails approximately match.

We can actually conclude that just taking a straight proportion of titles on sale in kindle format vs. all print books amazon sells is probably very nearly correct, mathmatically. 100k/2000k = 5%. And finally, 5% of 24 million is $1.2 million.

So my best guess is kindle content sales are around $1 million per month in round numbers. At $20 per customer per month that puts the number of kindles in the field at 50,000 in the first quarter.

This 50,000 figure is in the ballpark of estimates made by other analysts in other ways.

Your mileage may vary. Obviously a lot of assumptions are being made here. But I have other data that is also very consistent with these estimates.

-Steve P.