This is significant because the books are relatively new--Patterson's been accruing ebook sales in many formats on many books over many years. I've heard the announced Kindle numbers are just over 800,000. For Larsson to come in and do it with three books in a handful of years is a sign of the revolution.
By this time next year, I predict we will have a single book surpassing the million-Kindle mark. Possibly The Passage?
Speed Dating with the dead