The Kindle train keeps on rolling along, can anything stop it? In an app update for Android Amazon let loose the fact that there are now some 700,000 books for sale in the US Kindle store.

Why is that number astounding? Not because it is so large, that speaks for itself. No, the amazing fact is that just three weeks ago there were only 670,000 Kindle books for sale in the US. That suggests a rate of growth of some 10,000 books per week in the US Kindle store.

At more than 40,000 books a month, it will not take long for Kindle to have one million books for sale in the US. Expect Amazon to crow from the rooftops when that does happen.

Don’t forget, Kindle is expected to sell some 500 million dollars in books in the US this year. At 700,000 titles, that would suggest a revenue per book (averaged) of around $714 per book per year.