You might remember that a week after the Pro Drupal Development book shipped, it cracked the Amazon top-100. The book remains in the 4,000-5,000 rank at Amazon, where it has 28 reviews and an average of 5/5 stars.
Well, John VanDyk, co-author of the Pro Drupal Development book, wrote in to say that during the first 2.5 months (April, May, June) more than 6,000 copies were sold. And this number doesn't include sales data for July, August, September or October.
Without a doubt, these numbers are truly amazing. It is simply astounding to think that in a few short months one book has given the world 10,000 new or greatly improved Drupal developers. It blows my mind.